fix: Save session after OAuth callback before redirect

LOGIN LOOP FIXED! Session data now persists from callback to profile.

PROBLEM: Callback redirect was losing session data
- Callback sets session['user_id'] = 'qnib' ✅
- Redirects to /profile/ ➡️
- Profile checks session['user_id'] ❌ MISSING
- Redirects to /login → infinite loop 🔄

ROOT CAUSE: redirect() bypasses Flask's automatic session save
Same issue as login endpoint - manual save needed

SOLUTION: session_interface.save_session() before redirect
Now user_id persists across redirect

BEFORE:
/callback → sets user_id → redirect (session lost)
/profile/ → no user_id → redirect /login
/login → new OAuth flow → loop

AFTER:
/callback → sets user_id → manual save → redirect
/profile/ → user_id present → show profile ✅

src/routes/auth.py

@@ -138,7 +138,14 @@ def callback():
         session["access_token"] = token.get("access_token")
 
         flash(f"Welcome back, {display_name}!", "success")
-        return redirect(url_for("profile.view_profile"))
+        
+        # CRITICAL: Manually save session before redirect
+        # Same issue as login - redirect response bypasses automatic session save
+        response = redirect(url_for("profile.view_profile"))
+        from flask import current_app
+        current_app.session_interface.save_session(current_app, session, response)
+        logger.info(f"[OAUTH] Session saved with user_id={user_id}, redirecting to profile")
+        return response
 
     except Exception as e:
         # Log the full error for debugging