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>

templates/index.html

@@ -14158,7 +14158,7 @@
                     <li>Real-time ISS tracking map with ground track overlay</li>
                     <li>Next-pass countdown with elevation and duration predictions</li>
                     <li>Optional Doppler shift compensation for improved reception</li>
-                    <li>Requires: <code>slowrx</code> decoder + RTL-SDR</li>
+                    <li>Requires: RTL-SDR (no external decoder needed - built-in Python SSTV decoder)</li>
                 </ul>
 
                 <h3>HF SSTV Mode</h3>
@@ -14168,7 +14168,7 @@
                     <li>Supports USB, LSB, and FM demodulation modes</li>
                     <li>Auto-detects correct modulation when selecting a preset frequency</li>
                     <li>HF frequencies (below 30 MHz) require an upconverter with RTL-SDR</li>
-                    <li>Requires: <code>slowrx</code> decoder + RTL-SDR (+ upconverter for HF)</li>
+                    <li>Requires: RTL-SDR (+ upconverter for HF, no external decoder needed)</li>
                 </ul>
             </div>