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/user_detail.py

@@ -0,0 +1,72 @@
+from datetime import datetime
+from typing import Optional
+from uuid import UUID
+from pydantic import BaseModel
+
+
+class AccountSummary(BaseModel):
+    id: UUID
+    name: str
+    display_code: Optional[str] = None
+
+    model_config = {"from_attributes": True}
+
+
+class SubscriptionSummary(BaseModel):
+    id: UUID
+    plan: str
+    status: str
+    current_period_start: Optional[datetime] = None
+    current_period_end: Optional[datetime] = None
+
+    model_config = {"from_attributes": True}
+
+
+class SessionSummary(BaseModel):
+    id: UUID
+    tree_name: Optional[str] = None
+    started_at: datetime
+    completed_at: Optional[datetime] = None
+    outcome: Optional[str] = None
+
+    model_config = {"from_attributes": True}
+
+
+class AuditLogSummary(BaseModel):
+    id: UUID
+    action: str
+    resource_type: Optional[str] = None
+    resource_id: Optional[str] = None
+    created_at: datetime
+    details: Optional[dict] = None
+
+    model_config = {"from_attributes": True}
+
+
+class InviteCodeUsedSummary(BaseModel):
+    code: str
+    assigned_plan: str
+    trial_duration_days: Optional[int] = None
+    created_by_email: Optional[str] = None
+
+    model_config = {"from_attributes": True}
+
+
+class UserDetailResponse(BaseModel):
+    id: UUID
+    email: str
+    full_name: Optional[str] = None
+    role: str
+    is_active: bool
+    is_super_admin: bool
+    is_team_admin: bool
+    created_at: datetime
+
+    account: Optional[AccountSummary] = None
+    subscription: Optional[SubscriptionSummary] = None
+    invite_code_used: Optional[InviteCodeUsedSummary] = None
+
+    recent_sessions: list[SessionSummary] = []
+    total_sessions: int = 0
+    recent_audit_logs: list[AuditLogSummary] = []
+    total_audit_logs: int = 0