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/core/config.py

@@ -52,6 +52,15 @@ class Settings(BaseSettings):
     # Registration
     REQUIRE_INVITE_CODE: bool = True  # Set to False to allow open registration
 
+    # Email (Resend)
+    RESEND_API_KEY: Optional[str] = None
+    FROM_EMAIL: str = "ResolutionFlow <invites@resolutionflow.com>"
+
+    @property
+    def email_enabled(self) -> bool:
+        """Check if email sending is configured."""
+        return self.RESEND_API_KEY is not None
+
     # Stripe
     STRIPE_SECRET_KEY: Optional[str] = None
     STRIPE_PUBLISHABLE_KEY: Optional[str] = None