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",

backend/app/models/device.py

@@ -0,0 +1,27 @@
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import DateTime, ForeignKey, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db import Base
+
+
+class Device(Base):
+    """A paired hardware sensor node (ESP32-P4 etc.), belonging to exactly
+    one user. Authenticates telemetry via a bearer token whose hash — never
+    the raw value — is stored here, identical convention to
+    `AuthSession.token_hash` (see `app/models/auth_session.py`)."""
+
+    __tablename__ = "devices"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    user_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"), index=True)
+    name: Mapped[str] = mapped_column(String(64))
+    token_hash: Mapped[str] = mapped_column(String(64), unique=True, index=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+    last_seen_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )