Merge Workstream G: device pairing + ingestion + live broadcast

Resolved conflict in main.py: combined both workstreams' router imports
and registrations (device_router from G, inventory_router from C).
143/143 backend tests pass after cleaning stray pollution from an
earlier parallel workstream run against the shared test DB.

backend/app/models/__init__.py

@@ -1,5 +1,6 @@
 from app.models.auth_session import AuthSession
 from app.models.contact_session import ContactSession
+from app.models.device import Device
 from app.models.entity import Entity
 from app.models.entity_sighting import EntitySighting
 from app.models.event import Event
@@ -13,6 +14,7 @@ __all__ = [
     "User",
     "AuthSession",
     "ContactSession",
+    "Device",
     "Entity",
     "EntitySighting",
     "Event",