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/static_data_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"
 )
 
@@ -34,37 +35,37 @@ func (h *StaticDataHandler) GetStaticData(c *gin.Context) {
 	// Get all lookup data
 	residenceTypes, err := h.residenceService.GetResidenceTypes()
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch residence types"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": i18n.LocalizedMessage(c, "error.failed_to_fetch_residence_types")})
 		return
 	}
 
 	taskCategories, err := h.taskService.GetCategories()
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch task categories"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": i18n.LocalizedMessage(c, "error.failed_to_fetch_task_categories")})
 		return
 	}
 
 	taskPriorities, err := h.taskService.GetPriorities()
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch task priorities"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": i18n.LocalizedMessage(c, "error.failed_to_fetch_task_priorities")})
 		return
 	}
 
 	taskFrequencies, err := h.taskService.GetFrequencies()
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch task frequencies"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": i18n.LocalizedMessage(c, "error.failed_to_fetch_task_frequencies")})
 		return
 	}
 
 	taskStatuses, err := h.taskService.GetStatuses()
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch task statuses"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": i18n.LocalizedMessage(c, "error.failed_to_fetch_task_statuses")})
 		return
 	}
 
 	contractorSpecialties, err := h.contractorService.GetSpecialties()
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch contractor specialties"})
+		c.JSON(http.StatusInternalServerError, gin.H{"error": i18n.LocalizedMessage(c, "error.failed_to_fetch_contractor_specialties")})
 		return
 	}
 
@@ -83,7 +84,7 @@ func (h *StaticDataHandler) GetStaticData(c *gin.Context) {
 // Kept for API compatibility with mobile clients
 func (h *StaticDataHandler) RefreshStaticData(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{
-		"message": "Static data refreshed",
+		"message": i18n.LocalizedMessage(c, "message.static_data_refreshed"),
 		"status":  "success",
 	})
 }