fix: pass CFAI_ACCOUNT_ID/CFAI_API_TOKEN into backend and runner containers

Both vars were in Doppler but never forwarded via docker-compose.yml, causing
the image generation handler to return 503 "CFAI_ACCOUNT_ID/CFAI_API_TOKEN missing".

docker-compose.yml

@@ -19,6 +19,9 @@ x-infra-env: &infra-env
   MEILI_API_KEY: "${MEILI_MASTER_KEY}"
   # Valkey
   VALKEY_ADDR: "valkey:6379"
+  # Cloudflare AI (TTS + image generation)
+  CFAI_ACCOUNT_ID: "${CFAI_ACCOUNT_ID}"
+  CFAI_API_TOKEN: "${CFAI_API_TOKEN}"
 
 services:
   # ─── MinIO (object storage: chapters, audio, avatars, browse) ────────────────