feat: admin invite codes with plan assignment + user detail page

- Migration 030: add email, assigned_plan, trial_duration_days, email_sent_at
  to invite_codes with CHECK constraints
- Resend email integration (graceful degradation when API key not set)
- Invite codes now support plan assignment (free/pro/team) and trial duration (1-90 days)
- Registration applies invite code plan/trial to new subscription
- Auto-downgrade expired trials on authenticated access
- Enriched GET /admin/users/{id} with account, subscription, sessions, audit logs
- New endpoints: PUT /admin/users/{id}/subscription/plan and extend-trial
- Frontend: enhanced invite codes page with email, plan, trial fields
- Frontend: new user detail page at /admin/users/:userId
- Fixed API path drift: /invite-codes -> /invites
- 11 new backend tests, 416 total passing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/deps.py

@@ -65,14 +65,36 @@ async def get_refresh_token_payload(
 
 
 async def get_current_active_user(
-    current_user: Annotated[User, Depends(get_current_user)]
+    current_user: Annotated[User, Depends(get_current_user)],
+    db: Annotated[AsyncSession, Depends(get_db)],
 ) -> User:
-    """Ensure user is active (not disabled)."""
+    """Ensure user is active (not disabled). Auto-downgrades expired trials."""
     if not current_user.is_active:
         raise HTTPException(
             status_code=status.HTTP_403_FORBIDDEN,
             detail="Account has been deactivated"
         )
+
+    # Lightweight trial expiry check
+    if current_user.account_id:
+        from app.models.subscription import Subscription
+        from datetime import datetime, timezone
+        result = await db.execute(
+            select(Subscription).where(Subscription.account_id == current_user.account_id)
+        )
+        subscription = result.scalar_one_or_none()
+        if (
+            subscription
+            and subscription.status == "trialing"
+            and subscription.current_period_end
+            and subscription.current_period_end < datetime.now(timezone.utc)
+        ):
+            subscription.plan = "free"
+            subscription.status = "active"
+            subscription.current_period_end = None
+            subscription.current_period_start = None
+            await db.commit()
+
     return current_user