chore: replace console.log with structured logging (#141)

- Add startup logger (utils/logger.ts) with LOG_LEVEL support
- Add ServiceLogger type for scheduler functions
- Replace all console.log calls with leveled log methods
- Downgrade verbose scheduler info logs to debug level
- Remove unnecessary console.log in auth plugin

backend/src/plugins/auth.ts

@@ -37,7 +37,6 @@ export async function getAnonymousUserId(): Promise<number> {
   `);
 
 	anonymousUserVerified = true;
-	console.log(`Created anonymous user with fixed ID ${ANONYMOUS_USER_ID} for no-auth mode`);
 
 	return ANONYMOUS_USER_ID;
 }