feat: add multi-step user onboarding wizard

- 5-step wizard: Welcome → Profile → Plan → Storage → All Set!
- New migration 017: onboarding_completed, contact_email, preferred_destination fields
- REST API at /api/onboarding/{status,profile,plan,storage,complete}
- GET /onboarding view with configured-destinations helper
- OAuth callback redirects first-time users to onboarding
- 16 unit tests for all endpoints; 65 total tests pass

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/auth.py

@@ -165,7 +165,7 @@ async def oauth_callback(request: Request, db: Session = Depends(get_db)):
         _ensure_user_profile(db, user_data)
 
         # Log the successful authentication
-        logger.info(f"[SECURITY] OAUTH_LOGIN_SUCCESS user={user_data.get('email', 'unknown')} admin={is_admin}")
+        logger.info("[SECURITY] OAUTH_LOGIN_SUCCESS user=%s admin=%s", user_data.get("email", "unknown"), is_admin)
 
         # Redirect first-time users to onboarding
         user_id = (