fix(drone): resolve critical pipeline, frontend, and input validation issues

Data pipeline (critical): scanners/detectors now write to a separate _obs_queue;
a relay thread reads observations and calls correlator.process(), which emits
processed DroneContact dicts to drone_queue for SSE. Without this the SSE stream
received raw unserializable dataclass objects causing JSON errors.

Frontend (critical):
- Add droneContactList container to drone.html so contact cards render
- Add droneMap container and initialize Leaflet in drone.js init()
- Define dsc-distress-pulse keyframes in drone.css (was referenced but missing)
- Fix SSE reconnect: null _sse before setTimeout to prevent _connectSSE no-op loop

Other fixes:
- Validate rtl_sdr_index with validate_device_index(), return 400 on bad input
- Move _ensure_workers() inside _drone_lock to prevent double-initialization race
- Add double-call guard to RemoteIDScanner.start()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

utils/drone/remote_id.py

@@ -98,6 +98,8 @@ class RemoteIDScanner:
             elt = elt.payload if hasattr(elt, "payload") and isinstance(elt.payload, Dot11Elt) else None
 
     def start(self, wifi_iface: str | None = None) -> None:
+        if self._running:
+            return
         self._running = True
         if SCAPY_AVAILABLE and wifi_iface:
             try: