Add real-time WebSocket waterfall with I/Q capture and server-side FFT

Replace the batch rtl_power SSE pipeline with continuous I/Q streaming
via WebSocket for smooth ~25fps waterfall display. The server captures
raw I/Q samples (rtl_sdr/rx_sdr), computes Hann-windowed FFT, and
sends compact binary frames (1035 bytes vs ~15KB JSON, 93% reduction).
Client falls back to existing SSE path if WebSocket is unavailable.

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

app.py

@@ -869,6 +869,14 @@ def main() -> None:
     except ImportError as e:
         print(f"KiwiSDR audio proxy disabled: {e}")
 
+    # Initialize WebSocket for waterfall streaming
+    try:
+        from routes.waterfall_websocket import init_waterfall_websocket
+        init_waterfall_websocket(app)
+        print("WebSocket waterfall streaming enabled")
+    except ImportError as e:
+        print(f"WebSocket waterfall disabled: {e}")
+
     print(f"Open http://localhost:{args.port} in your browser")
     print()
     print("Press Ctrl+C to stop")