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>

pyproject.toml

@@ -57,6 +57,7 @@ optionals = [
     "scipy>=1.10.0",
     "qrcode[pil]>=7.4",
     "numpy>=1.24.0",
+    "Pillow>=9.0.0",
     "meshtastic>=2.0.0",
     "psycopg2-binary>=2.9.9",
     "scapy>=2.4.5",