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>

static/css/modes/drone.css

@@ -69,6 +69,18 @@
     color: var(--accent-red);
 }
 
+.drone-map {
+    height: 280px;
+    border-radius: 4px;
+    border: 1px solid var(--border-color);
+    margin: 0 12px 12px;
+}
+
 .drone-marker-high-risk {
     animation: dsc-distress-pulse 1.5s infinite;
 }
+
+@keyframes dsc-distress-pulse {
+    0%, 100% { opacity: 1; transform: scale(1); }
+    50% { opacity: 0.4; transform: scale(1.4); }
+}