refactor: consolidate lang into constants, rename services to email

- Merge lang package into constants (add IsValidLang, ValidateLang, AllLangs)
- Rename internal/services to internal/email for consistency with pdf package
- Rename types to avoid redundancy: EmailService→Service, EmailConfig→Config
- Update all imports and references across codebase
- Delete internal/lang directory (functions moved to constants)

internal/middleware/security.go

@@ -208,7 +208,7 @@ func (rl *RateLimiter) allow(ip string) bool {
 
 // cleanup removes expired entries periodically
 func (rl *RateLimiter) cleanup() {
-	ticker := time.NewTicker(1 * time.Minute)
+	ticker := time.NewTicker(c.RateLimitGeneralCleanupPeriod)
 	defer ticker.Stop()
 
 	for range ticker.C {