feat(13-03): add link service, schemas, router, and wire subscribers into lifespan

- LinkResponse/UnknownClientResponse Pydantic schemas with from_attributes
- Link service with get_links, get_device_links, get_site_links, get_unknown_clients
- Unknown clients query uses DISTINCT ON for latest registration per MAC
- 4 REST endpoints: tenant links, device links, site links, unknown clients
- Interface and link discovery subscribers wired into FastAPI lifespan start/stop
- Links router registered at /api prefix

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/link.py

@@ -0,0 +1,56 @@
+"""Pydantic schemas for Link and Unknown Client endpoints."""
+
+import uuid
+from datetime import datetime
+from typing import Optional
+
+from pydantic import BaseModel, ConfigDict
+
+
+class LinkResponse(BaseModel):
+    """Single wireless link between an AP and CPE device."""
+
+    id: uuid.UUID
+    ap_device_id: uuid.UUID
+    cpe_device_id: uuid.UUID
+    ap_hostname: str | None = None
+    cpe_hostname: str | None = None
+    interface: str | None = None
+    client_mac: str
+    signal_strength: int | None = None
+    tx_ccq: int | None = None
+    tx_rate: str | None = None
+    rx_rate: str | None = None
+    state: str
+    missed_polls: int
+    discovered_at: datetime
+    last_seen: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class LinkListResponse(BaseModel):
+    """List of wireless links with total count."""
+
+    items: list[LinkResponse]
+    total: int
+
+
+class UnknownClientResponse(BaseModel):
+    """A wireless client whose MAC does not resolve to any known device interface."""
+
+    mac_address: str
+    interface: str | None = None
+    signal_strength: int | None = None
+    tx_rate: str | None = None
+    rx_rate: str | None = None
+    last_seen: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class UnknownClientListResponse(BaseModel):
+    """List of unknown clients with total count."""
+
+    items: list[UnknownClientResponse]
+    total: int