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/router/router.go

@@ -11,6 +11,7 @@ import (
 	"github.com/treytartt/casera-api/internal/admin"
 	"github.com/treytartt/casera-api/internal/config"
 	"github.com/treytartt/casera-api/internal/handlers"
+	"github.com/treytartt/casera-api/internal/i18n"
 	"github.com/treytartt/casera-api/internal/middleware"
 	"github.com/treytartt/casera-api/internal/push"
 	"github.com/treytartt/casera-api/internal/repositories"
@@ -48,6 +49,7 @@ func SetupRouter(deps *Dependencies) *gin.Engine {
 	r.Use(utils.GinRecovery())
 	r.Use(utils.GinLogger())
 	r.Use(corsMiddleware(cfg))
+	r.Use(i18n.Middleware())
 
 	// Health check endpoint (no auth required)
 	r.GET("/api/health/", healthCheck)