Redact database URL passwords from logs

- Add SanitizeDatabaseURL() function to config package that redacts passwords from database connection URLs
- Handles various URL formats: postgresql, mysql, etc.
- Correctly handles passwords containing special characters including @ symbols by using LastIndex
- Update startup logging in cmd/server/main.go to use sanitized database URL
- Add comprehensive tests covering various URL formats and edge cases

This ensures sensitive database credentials are not exposed in application logs.

cmd/server/main.go

@@ -103,7 +103,7 @@ func main() {
 			if err != nil {
 				logger.Fatalf("Failed to create database key store: %v", err)
 			}
-			logger.Infof("Connected to authentication database: %s", cfg.Auth.Database.URL)
+			logger.Infof("Connected to authentication database: %s", config.SanitizeDatabaseURL(cfg.Auth.Database.URL))
 		} else {
 			logger.Warn("No database configured for authentication - API keys will only be stored in memory")
 		}