fix(tts): fix pocket-tts voices missing in UI and 500 on first TTS enqueue

- Add POCKET_TTS_URL env to backend service in docker-compose.yml so
  pocket-tts voices appear in the voice selector (Doppler secret existed
  but the env var was never passed to the container)
- Fix GetAudioTask PocketBase filter using %q (double-quotes) instead of
  single-quoted string, causing the duplicate-task guard to always miss
- Fix AudioPlayer double-POST: GET /api/presign/audio already enqueues
  TTS internally on 404; AudioPlayer now skips the redundant POST and
  polls directly, eliminating the 500 from the PB unique-key conflict

docker-compose.yml

@@ -154,12 +154,13 @@ services:
     # No public port — all traffic is routed via Caddy.
     expose:
       - "8080"
-    environment:
+     environment:
       <<: *infra-env
       BACKEND_HTTP_ADDR: ":8080"
       LOG_LEVEL: "${LOG_LEVEL}"
       KOKORO_URL: "${KOKORO_URL}"
       KOKORO_VOICE: "${KOKORO_VOICE}"
+      POCKET_TTS_URL: "${POCKET_TTS_URL}"
       GLITCHTIP_DSN: "${GLITCHTIP_DSN}"
       OTEL_EXPORTER_OTLP_ENDPOINT: "${OTEL_EXPORTER_OTLP_ENDPOINT}"
       OTEL_SERVICE_NAME: "backend"