feat: add realtime GNSS monitor with telemetry websocket

backend/app/api/routes/telemetry.py

@@ -0,0 +1,63 @@
+from __future__ import annotations
+
+import asyncio
+from typing import Any
+
+from fastapi import APIRouter, HTTPException, Query, WebSocket, status
+from fastapi.encoders import jsonable_encoder
+from fastapi.websockets import WebSocketDisconnect
+from sqlmodel import Session
+
+from app.api.deps import CurrentUser, SessionDep, resolve_user_from_token
+from app.core.db import engine
+from app.mqtt.telemetry import build_telemetry_snapshot
+
+router = APIRouter(prefix="/telemetry", tags=["telemetry"])
+STREAM_INTERVAL_SECONDS = 1.0
+
+
+@router.get("/latest")
+def read_telemetry_latest(
+    session: SessionDep,
+    _current_user: CurrentUser,
+    device_id: str | None = None,
+    limit: int = Query(default=50, ge=1, le=200),
+) -> dict[str, Any]:
+    """
+    Return the latest parsed GNSS telemetry snapshot.
+    """
+    return build_telemetry_snapshot(session=session, device_id=device_id, limit=limit)
+
+
+@router.websocket("/ws")
+async def telemetry_websocket(
+    websocket: WebSocket,
+    token: str | None = Query(default=None),
+    device_id: str | None = Query(default=None),
+    limit: int = Query(default=50, ge=1, le=200),
+) -> None:
+    if not token:
+        await websocket.close(code=status.WS_1008_POLICY_VIOLATION)
+        return
+
+    with Session(engine) as session:
+        try:
+            resolve_user_from_token(session=session, token=token)
+        except HTTPException:
+            await websocket.close(code=status.WS_1008_POLICY_VIOLATION)
+            return
+
+    await websocket.accept()
+
+    try:
+        while True:
+            with Session(engine) as session:
+                payload = build_telemetry_snapshot(
+                    session=session,
+                    device_id=device_id,
+                    limit=limit,
+                )
+            await websocket.send_json(jsonable_encoder(payload))
+            await asyncio.sleep(STREAM_INTERVAL_SECONDS)
+    except (WebSocketDisconnect, RuntimeError):
+        return