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>

cmd/api/main.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/treytartt/casera-api/internal/config"
 	"github.com/treytartt/casera-api/internal/database"
+	"github.com/treytartt/casera-api/internal/i18n"
 	"github.com/treytartt/casera-api/internal/push"
 	"github.com/treytartt/casera-api/internal/router"
 	"github.com/treytartt/casera-api/internal/services"
@@ -31,6 +32,13 @@ func main() {
 	// Initialize logger
 	utils.InitLogger(cfg.Server.Debug)
 
+	// Initialize i18n
+	if err := i18n.Init(); err != nil {
+		log.Warn().Err(err).Msg("Failed to initialize i18n - using English only")
+	} else {
+		log.Info().Strs("languages", i18n.SupportedLanguages).Msg("i18n initialized")
+	}
+
 	log.Info().
 		Bool("debug", cfg.Server.Debug).
 		Int("port", cfg.Server.Port).