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/schemas/schemas.py

@@ -176,6 +176,9 @@ class UserBase(BaseModel):
 class UserCreate(UserBase):
     password: Optional[str] = None
     role_id: Optional[int] = None
+    # Agent binding (both must be provided or both omitted)
+    agent_id: Optional[str] = None
+    claw_identifier: Optional[str] = None
 
 
 class UserUpdate(BaseModel):
@@ -192,6 +195,7 @@ class UserResponse(UserBase):
     is_admin: bool
     role_id: Optional[int] = None
     role_name: Optional[str] = None
+    agent_id: Optional[str] = None
     created_at: datetime
     
     class Config:
@@ -389,6 +393,47 @@ class ProposalAcceptResponse(ProposalResponse):
         from_attributes = True
 
 
+# ---------------------------------------------------------------------------
+# Agent schemas (BE-CAL-003)
+# ---------------------------------------------------------------------------
+
+class AgentStatusEnum(str, Enum):
+    IDLE = "idle"
+    ON_CALL = "on_call"
+    BUSY = "busy"
+    EXHAUSTED = "exhausted"
+    OFFLINE = "offline"
+
+
+class ExhaustReasonEnum(str, Enum):
+    RATE_LIMIT = "rate_limit"
+    BILLING = "billing"
+
+
+class AgentResponse(BaseModel):
+    """Read-only representation of an Agent."""
+    id: int
+    user_id: int
+    agent_id: str
+    claw_identifier: str
+    status: AgentStatusEnum
+    last_heartbeat: Optional[datetime] = None
+    exhausted_at: Optional[datetime] = None
+    recovery_at: Optional[datetime] = None
+    exhaust_reason: Optional[ExhaustReasonEnum] = None
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class AgentStatusUpdate(BaseModel):
+    """Payload for updating an agent's runtime status."""
+    status: AgentStatusEnum
+    exhaust_reason: Optional[ExhaustReasonEnum] = None
+    recovery_at: Optional[datetime] = None
+
+
 # Backward-compatible aliases
 ProposeStatusEnum = ProposalStatusEnum
 ProposeBase = ProposalBase