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/invite_code.py

@@ -1,13 +1,22 @@
 from datetime import datetime
-from typing import Optional
+from typing import Literal, Optional
 from uuid import UUID
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, EmailStr, Field, model_validator
 
 
 class InviteCodeCreate(BaseModel):
     """Schema for creating a new invite code."""
     expires_at: Optional[datetime] = Field(None, description="Optional expiration time")
     note: Optional[str] = Field(None, max_length=255, description="Note about who this code is for")
+    email: Optional[EmailStr] = Field(None, description="Recipient email for invite delivery")
+    assigned_plan: Literal["free", "pro", "team"] = Field("free", description="Plan to assign on registration")
+    trial_duration_days: Optional[int] = Field(None, ge=1, le=90, description="Trial duration in days (1-90)")
+
+    @model_validator(mode="after")
+    def free_plan_no_trial(self):
+        if self.assigned_plan == "free" and self.trial_duration_days is not None:
+            raise ValueError("Free plan cannot have a trial duration")
+        return self
 
 
 class InviteCodeResponse(BaseModel):
@@ -23,6 +32,12 @@ class InviteCodeResponse(BaseModel):
     is_used: bool
     is_expired: bool
     is_valid: bool
+    email: Optional[str] = None
+    assigned_plan: str = "free"
+    trial_duration_days: Optional[int] = None
+    email_sent_at: Optional[datetime] = None
+    has_trial: bool = False
+    email_sent: bool = False
 
     class Config:
         from_attributes = True