Replace broken slowrx dependency with pure Python SSTV decoder

slowrx is a GTK GUI app that doesn't support CLI usage, so the SSTV
decoder was silently failing. This replaces it with a pure Python
implementation using numpy and Pillow that supports Robot36/72,
Martin1/2, Scottie1/2, and PD120/180 modes via VIS header auto-detection.

Key implementation details:
- Generalized Goertzel (DTFT) for exact-frequency tone detection
- Vectorized batch Goertzel for real-time pixel decoding performance
- Overlapping analysis windows for short-window frequency estimation
- VIS header detection state machine with parity validation
- Per-line sync re-synchronization for drift tolerance

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

routes/sstv.py

@@ -94,7 +94,7 @@ def start_decoder():
     if not is_sstv_available():
         return jsonify({
             'status': 'error',
-            'message': 'SSTV decoder not available. Install slowrx: apt install slowrx'
+            'message': 'SSTV decoder not available. Install numpy and Pillow: pip install numpy Pillow'
         }), 400
 
     decoder = get_sstv_decoder()

routes/sstv_general.py

@@ -99,7 +99,7 @@ def start_decoder():
     if decoder.decoder_available is None:
         return jsonify({
             'status': 'error',
-            'message': 'SSTV decoder not available. Install slowrx: apt install slowrx',
+            'message': 'SSTV decoder not available. Install numpy and Pillow: pip install numpy Pillow',
         }), 400
 
     if decoder.is_running: