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/upload_handler.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 
+	"github.com/treytartt/casera-api/internal/i18n"
 	"github.com/treytartt/casera-api/internal/services"
 )
 
@@ -23,7 +24,7 @@ func NewUploadHandler(storageService *services.StorageService) *UploadHandler {
 func (h *UploadHandler) UploadImage(c *gin.Context) {
 	file, err := c.FormFile("file")
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "No file provided"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.no_file_provided")})
 		return
 	}
 
@@ -44,7 +45,7 @@ func (h *UploadHandler) UploadImage(c *gin.Context) {
 func (h *UploadHandler) UploadDocument(c *gin.Context) {
 	file, err := c.FormFile("file")
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "No file provided"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.no_file_provided")})
 		return
 	}
 
@@ -62,7 +63,7 @@ func (h *UploadHandler) UploadDocument(c *gin.Context) {
 func (h *UploadHandler) UploadCompletion(c *gin.Context) {
 	file, err := c.FormFile("file")
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "No file provided"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.no_file_provided")})
 		return
 	}
 
@@ -92,5 +93,5 @@ func (h *UploadHandler) DeleteFile(c *gin.Context) {
 		return
 	}
 
-	c.JSON(http.StatusOK, gin.H{"message": "File deleted successfully"})
+	c.JSON(http.StatusOK, gin.H{"message": i18n.LocalizedMessage(c, "message.file_deleted")})
 }