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/models/invite_code.py

@@ -46,6 +46,13 @@ class InviteCode(Base):
         DateTime(timezone=True),
         nullable=True
     )
+    email: Mapped[Optional[str]] = mapped_column(String(255), nullable=True, index=True)
+    assigned_plan: Mapped[str] = mapped_column(String(50), nullable=False, server_default="free")
+    trial_duration_days: Mapped[Optional[int]] = mapped_column(nullable=True)
+    email_sent_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True
+    )
     note: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
@@ -84,3 +91,11 @@ class InviteCode(Base):
     def is_valid(self) -> bool:
         """Check if the invite code is valid (not used and not expired)."""
         return not self.is_used and not self.is_expired
+
+    @property
+    def has_trial(self) -> bool:
+        return self.trial_duration_days is not None and self.trial_duration_days > 0
+
+    @property
+    def email_sent(self) -> bool:
+        return self.email_sent_at is not None