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>

utils/sstv/__init__.py

@@ -0,0 +1,33 @@
+"""SSTV (Slow-Scan Television) decoder package.
+
+Pure Python SSTV decoder using Goertzel-based DSP for VIS header detection
+and scanline-by-scanline image decoding. Supports Robot36/72, Martin1/2,
+Scottie1/2, and PD120/180 modes.
+
+Replaces the external slowrx dependency with numpy/scipy + Pillow.
+"""
+
+from .constants import ISS_SSTV_FREQ, SSTV_MODES
+from .sstv_decoder import (
+    DecodeProgress,
+    DopplerInfo,
+    DopplerTracker,
+    SSTVDecoder,
+    SSTVImage,
+    get_general_sstv_decoder,
+    get_sstv_decoder,
+    is_sstv_available,
+)
+
+__all__ = [
+    'DecodeProgress',
+    'DopplerInfo',
+    'DopplerTracker',
+    'ISS_SSTV_FREQ',
+    'SSTV_MODES',
+    'SSTVDecoder',
+    'SSTVImage',
+    'get_general_sstv_decoder',
+    'get_sstv_decoder',
+    'is_sstv_available',
+]