feat(users): PATCH /users/{id}/bind-agent to backfill agents row

Companion endpoint for the cli's upcoming `hf user bind-agent` subcommand.
Lets admin retroactively bind an existing user to (agent_id,
claw_identifier) when that user was created before `hf user create`
supported the binding flags (i.e. all of zhi/lyn/mirror/sherlock/orion/
nav on prod today — agents table has 0 rows even though their user rows
exist).

Schema:
  PATCH /users/{identifier}/bind-agent
  body: {agent_id: str, claw_identifier: str}  // both required
  perm: account.create (admin auto)            // same as POST /users

Behaviour:
  * idempotent: re-bind to the same (agent_id, claw_identifier) → 200
    no-op, no extra row
  * 409 if user is already bound to a different pair
  * 409 if requested agent_id is already in use by another user
  * creates the agents row inline; subsequent /schedule-types/agent/
    {agent_id}/assign etc. then work normally

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/schemas/schemas.py

@@ -1,4 +1,4 @@
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 from typing import Optional, List
 from datetime import datetime, time
 from enum import Enum
@@ -186,6 +186,19 @@ class UserUpdate(BaseModel):
     discord_user_id: Optional[str] = None
 
 
+class UserBindAgentRequest(BaseModel):
+    """Request body for PATCH /users/{identifier}/bind-agent.
+
+    Binds an existing user to (agent_id, claw_identifier) by inserting a
+    row in the `agents` table. Both fields required (mirrors the
+    create-time invariant in UserCreate). Idempotent: re-binding the same
+    user to the same (agent_id, claw_identifier) returns the existing
+    Agent row instead of 409.
+    """
+    agent_id: str = Field(..., min_length=1, max_length=128)
+    claw_identifier: str = Field(..., min_length=1, max_length=128)
+
+
 class UserResponse(UserBase):
     id: int
     is_active: bool