fix(docker): restore STT (faster-whisper) on Python 3.14

Previously removed all voice deps from the runtime image because of the
numpy<2 / cp314 wheel chain. Actual upstream check (PyPI 2026-05-21)
shows the chain has resolved for the STT half:

- ctranslate2 v4.7.2 (2026-05-19) ships cp314 wheels
- faster-whisper v1.2.1 is pure Python and works on any supported runtime
- onnxruntime v1.26.0 has cp314 wheels (not used here but shared with
  the upcoming piper-tts install)

The blocker was kokoro, not the whole stack. Kokoro has been stale
upstream since April 2025 with a `requires_python='<3.13'` pin; that's
being replaced separately with piper-tts.

This commit restores ONLY STT — faster-whisper + soundfile. No torch
(ctranslate2 does its own CPU inference), no kokoro, no spacy. Image
add: ~150 MB.

Voice code is lazily guarded; STT now works when VOICE_ENABLED=true.
TTS still fails gracefully (kokoro import error logged, voice degrades)
until the piper-tts swap lands.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -17,13 +17,14 @@ COPY src/ src/
 RUN --mount=type=cache,target=/root/.cache/pip \
     pip install .
 
-# Voice deps (faster-whisper, kokoro, torch, soundfile, spacy) are NOT
-# installed in the production image. They added ~2 GB and pulled numpy<2,
-# which has no cp314 wheel and broke the slim-image build.
-# Voice paths in code are lazily guarded; with VOICE_ENABLED=false (default),
-# the app starts cleanly. With voice enabled, missing imports log and
-# the feature gracefully degrades. To re-enable voice in a build, install
-# the `voice` extra: `pip install .[voice]` — and add an extras step here.
+# Speech-to-text (faster-whisper + soundfile). faster-whisper is pure
+# Python; the actual inference engine is ctranslate2 (C++) which has
+# cp314 wheels as of v4.7.2 (2026-05-19). No torch needed — ctranslate2
+# does its own CPU inference. Image add: ~150 MB.
+# TTS (piper-tts) is installed in a separate later step so STT can
+# stand alone if TTS becomes problematic.
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install faster-whisper soundfile
 
 # Build the fable-mcp wheel so it can be served for download
 COPY fable-mcp/ fable-mcp/