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/schemas/subscription.py

@@ -38,3 +38,15 @@ class SubscriptionDetails(BaseModel):
     subscription: SubscriptionResponse
     limits: PlanLimitsResponse
     usage: UsageResponse
+
+
+class SubscriptionPlanUpdate(BaseModel):
+    plan: str  # free, pro, team
+
+    model_config = {"json_schema_extra": {"examples": [{"plan": "pro"}]}}
+
+
+class ExtendTrialRequest(BaseModel):
+    days: int  # 1-90
+
+    model_config = {"json_schema_extra": {"examples": [{"days": 14}]}}