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>

templates/partials/modes/drone.html

@@ -39,4 +39,11 @@
             Non-compliant: <span id="droneNonCompliantCount" style="color:var(--accent-red);">0</span>
         </p>
     </div>
+
+    <div class="section">
+        <h3>Detected Contacts</h3>
+        <div id="droneContactList"></div>
+    </div>
+
+    <div id="droneMap" class="drone-map"></div>
 </div>