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/endpoints/invite.py

@@ -5,6 +5,8 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
 from app.core.database import get_db
 from app.core.rate_limit import limiter
+from app.core.audit import log_audit
+from app.core.email import EmailService
 from app.models.user import User
 from app.models.invite_code import InviteCode
 from app.schemas.invite_code import InviteCodeCreate, InviteCodeResponse, InviteCodeValidation
@@ -23,9 +25,35 @@ async def create_invite_code(
     invite_code = InviteCode(
         created_by_id=current_user.id,
         expires_at=invite_data.expires_at,
-        note=invite_data.note
+        note=invite_data.note,
+        email=invite_data.email,
+        assigned_plan=invite_data.assigned_plan,
+        trial_duration_days=invite_data.trial_duration_days,
     )
     db.add(invite_code)
+    await db.flush()
+
+    # Send invite email if email provided
+    email_sent = False
+    if invite_data.email:
+        email_sent = await EmailService.send_invite_email(
+            to_email=invite_data.email,
+            code=invite_code.code,
+            plan=invite_data.assigned_plan,
+            trial_days=invite_data.trial_duration_days,
+        )
+        if email_sent:
+            invite_code.email_sent_at = datetime.now(timezone.utc)
+
+    await log_audit(
+        db, current_user.id, "invite.create", "invite_code", invite_code.id,
+        {
+            "code": invite_code.code,
+            "plan": invite_data.assigned_plan,
+            "email": invite_data.email,
+            "email_sent": email_sent,
+        },
+    )
     await db.commit()
     await db.refresh(invite_code)