BE-CAL-003: Add Agent model with status/heartbeat/exhausted fields

- New app/models/agent.py with Agent model, AgentStatus & ExhaustReason enums
- Agent has 1-to-1 FK to User, unique agent_id (OpenClaw $AGENT_ID),
  claw_identifier (OpenClaw instance, convention-matches MonitoredServer.identifier)
- Status fields: status (idle/on_call/busy/exhausted/offline), last_heartbeat
- Exhausted tracking: exhausted_at, recovery_at, exhaust_reason (rate_limit/billing)
- User model: added 'agent' back-reference (uselist=False)
- Schemas: AgentResponse, AgentStatusUpdate, UserCreate now accepts agent_id+claw_identifier
- UserResponse: includes agent_id when agent is bound
- Users router: create_user creates Agent record when agent_id+claw_identifier provided
- Auto-migration: CREATE TABLE agents in _migrate_schema()
- Startup imports: agent and calendar models registered

app/models/models.py

@@ -81,6 +81,7 @@ class User(Base):
     owned_projects = relationship("Project", back_populates="owner")
     comments = relationship("Comment", back_populates="author")
     project_memberships = relationship("ProjectMember", back_populates="user")
+    agent = relationship("Agent", back_populates="user", uselist=False)
 
     @property
     def role_name(self):