Add comprehensive i18n localization support

- Add go-i18n package for internationalization
- Create i18n middleware to extract Accept-Language header
- Add translation files for en, es, fr, de, pt languages
- Localize all handler error messages and responses
- Add language context to all API handlers

Supported languages: English, Spanish, French, German, Portuguese

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/handlers/document_handler.go

@@ -12,6 +12,7 @@ import (
 	"github.com/shopspring/decimal"
 
 	"github.com/treytartt/casera-api/internal/dto/requests"
+	"github.com/treytartt/casera-api/internal/i18n"
 	"github.com/treytartt/casera-api/internal/middleware"
 	"github.com/treytartt/casera-api/internal/models"
 	"github.com/treytartt/casera-api/internal/services"
@@ -47,7 +48,7 @@ func (h *DocumentHandler) GetDocument(c *gin.Context) {
 	user := c.MustGet(middleware.AuthUserKey).(*models.User)
 	documentID, err := strconv.ParseUint(c.Param("id"), 10, 32)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid document ID"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.invalid_document_id")})
 		return
 	}
 
@@ -55,9 +56,9 @@ func (h *DocumentHandler) GetDocument(c *gin.Context) {
 	if err != nil {
 		switch {
 		case errors.Is(err, services.ErrDocumentNotFound):
-			c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
+			c.JSON(http.StatusNotFound, gin.H{"error": i18n.LocalizedMessage(c, "error.document_not_found")})
 		case errors.Is(err, services.ErrDocumentAccessDenied):
-			c.JSON(http.StatusForbidden, gin.H{"error": err.Error()})
+			c.JSON(http.StatusForbidden, gin.H{"error": i18n.LocalizedMessage(c, "error.document_access_denied")})
 		default:
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 		}
@@ -89,19 +90,19 @@ func (h *DocumentHandler) CreateDocument(c *gin.Context) {
 	if strings.HasPrefix(contentType, "multipart/form-data") {
 		// Parse multipart form
 		if err := c.Request.ParseMultipartForm(32 << 20); err != nil { // 32MB max
-			c.JSON(http.StatusBadRequest, gin.H{"error": "failed to parse multipart form: " + err.Error()})
+			c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.failed_to_parse_form")})
 			return
 		}
 
 		// Parse residence_id (required)
 		residenceIDStr := c.PostForm("residence_id")
 		if residenceIDStr == "" {
-			c.JSON(http.StatusBadRequest, gin.H{"error": "residence_id is required"})
+			c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.residence_id_required")})
 			return
 		}
 		residenceID, err := strconv.ParseUint(residenceIDStr, 10, 32)
 		if err != nil {
-			c.JSON(http.StatusBadRequest, gin.H{"error": "invalid residence_id"})
+			c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.invalid_residence_id")})
 			return
 		}
 		req.ResidenceID = uint(residenceID)
@@ -109,7 +110,7 @@ func (h *DocumentHandler) CreateDocument(c *gin.Context) {
 		// Parse title (required)
 		req.Title = c.PostForm("title")
 		if req.Title == "" {
-			c.JSON(http.StatusBadRequest, gin.H{"error": "title is required"})
+			c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.title_required")})
 			return
 		}
 
@@ -170,7 +171,7 @@ func (h *DocumentHandler) CreateDocument(c *gin.Context) {
 		if uploadedFile != nil && h.storageService != nil {
 			result, err := h.storageService.Upload(uploadedFile, "documents")
 			if err != nil {
-				c.JSON(http.StatusBadRequest, gin.H{"error": "failed to upload file: " + err.Error()})
+				c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.failed_to_upload_file")})
 				return
 			}
 			req.FileURL = result.URL
@@ -190,7 +191,7 @@ func (h *DocumentHandler) CreateDocument(c *gin.Context) {
 	response, err := h.documentService.CreateDocument(&req, user.ID)
 	if err != nil {
 		if errors.Is(err, services.ErrResidenceAccessDenied) {
-			c.JSON(http.StatusForbidden, gin.H{"error": err.Error()})
+			c.JSON(http.StatusForbidden, gin.H{"error": i18n.LocalizedMessage(c, "error.residence_access_denied")})
 			return
 		}
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
@@ -204,7 +205,7 @@ func (h *DocumentHandler) UpdateDocument(c *gin.Context) {
 	user := c.MustGet(middleware.AuthUserKey).(*models.User)
 	documentID, err := strconv.ParseUint(c.Param("id"), 10, 32)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid document ID"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.invalid_document_id")})
 		return
 	}
 
@@ -218,9 +219,9 @@ func (h *DocumentHandler) UpdateDocument(c *gin.Context) {
 	if err != nil {
 		switch {
 		case errors.Is(err, services.ErrDocumentNotFound):
-			c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
+			c.JSON(http.StatusNotFound, gin.H{"error": i18n.LocalizedMessage(c, "error.document_not_found")})
 		case errors.Is(err, services.ErrDocumentAccessDenied):
-			c.JSON(http.StatusForbidden, gin.H{"error": err.Error()})
+			c.JSON(http.StatusForbidden, gin.H{"error": i18n.LocalizedMessage(c, "error.document_access_denied")})
 		default:
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 		}
@@ -234,7 +235,7 @@ func (h *DocumentHandler) DeleteDocument(c *gin.Context) {
 	user := c.MustGet(middleware.AuthUserKey).(*models.User)
 	documentID, err := strconv.ParseUint(c.Param("id"), 10, 32)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid document ID"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.invalid_document_id")})
 		return
 	}
 
@@ -242,15 +243,15 @@ func (h *DocumentHandler) DeleteDocument(c *gin.Context) {
 	if err != nil {
 		switch {
 		case errors.Is(err, services.ErrDocumentNotFound):
-			c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
+			c.JSON(http.StatusNotFound, gin.H{"error": i18n.LocalizedMessage(c, "error.document_not_found")})
 		case errors.Is(err, services.ErrDocumentAccessDenied):
-			c.JSON(http.StatusForbidden, gin.H{"error": err.Error()})
+			c.JSON(http.StatusForbidden, gin.H{"error": i18n.LocalizedMessage(c, "error.document_access_denied")})
 		default:
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 		}
 		return
 	}
-	c.JSON(http.StatusOK, gin.H{"message": "Document deleted successfully"})
+	c.JSON(http.StatusOK, gin.H{"message": i18n.LocalizedMessage(c, "message.document_deleted")})
 }
 
 // ActivateDocument handles POST /api/documents/:id/activate/
@@ -258,7 +259,7 @@ func (h *DocumentHandler) ActivateDocument(c *gin.Context) {
 	user := c.MustGet(middleware.AuthUserKey).(*models.User)
 	documentID, err := strconv.ParseUint(c.Param("id"), 10, 32)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid document ID"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.invalid_document_id")})
 		return
 	}
 
@@ -266,15 +267,15 @@ func (h *DocumentHandler) ActivateDocument(c *gin.Context) {
 	if err != nil {
 		switch {
 		case errors.Is(err, services.ErrDocumentNotFound):
-			c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
+			c.JSON(http.StatusNotFound, gin.H{"error": i18n.LocalizedMessage(c, "error.document_not_found")})
 		case errors.Is(err, services.ErrDocumentAccessDenied):
-			c.JSON(http.StatusForbidden, gin.H{"error": err.Error()})
+			c.JSON(http.StatusForbidden, gin.H{"error": i18n.LocalizedMessage(c, "error.document_access_denied")})
 		default:
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 		}
 		return
 	}
-	c.JSON(http.StatusOK, gin.H{"message": "Document activated", "document": response})
+	c.JSON(http.StatusOK, gin.H{"message": i18n.LocalizedMessage(c, "message.document_activated"), "document": response})
 }
 
 // DeactivateDocument handles POST /api/documents/:id/deactivate/
@@ -282,7 +283,7 @@ func (h *DocumentHandler) DeactivateDocument(c *gin.Context) {
 	user := c.MustGet(middleware.AuthUserKey).(*models.User)
 	documentID, err := strconv.ParseUint(c.Param("id"), 10, 32)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid document ID"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.invalid_document_id")})
 		return
 	}
 
@@ -290,13 +291,13 @@ func (h *DocumentHandler) DeactivateDocument(c *gin.Context) {
 	if err != nil {
 		switch {
 		case errors.Is(err, services.ErrDocumentNotFound):
-			c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
+			c.JSON(http.StatusNotFound, gin.H{"error": i18n.LocalizedMessage(c, "error.document_not_found")})
 		case errors.Is(err, services.ErrDocumentAccessDenied):
-			c.JSON(http.StatusForbidden, gin.H{"error": err.Error()})
+			c.JSON(http.StatusForbidden, gin.H{"error": i18n.LocalizedMessage(c, "error.document_access_denied")})
 		default:
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 		}
 		return
 	}
-	c.JSON(http.StatusOK, gin.H{"message": "Document deactivated", "document": response})
+	c.JSON(http.StatusOK, gin.H{"message": i18n.LocalizedMessage(c, "message.document_deactivated"), "document": response})
 }