Phase 15: containerize Petal for the parodia.dev VPS

Deploy plumbing so Petal can run on the public VPS behind the Traefik
already on that box, with vLLM reached over headscale.

- Dockerfile: node build -> go build -> alpine runtime. CGO stays off
  (modernc SQLite is pure Go), so the runtime layer exists only for
  ffmpeg (read-aloud transcodes Piper's WAV) and tzdata (the companion's
  bedtime nag and night mode read the local clock). Runs as uid 10001
  with /data as the single writable mount.

- docker-compose.yml: Traefik labels following this host's convention
  (external `traefik` network, `web-secure` entrypoint, `default` cert
  resolver). Petal publishes no host port. ./data is a bind mount, not a
  named volume, so the nightly backup and a restore are reachable from
  the host.

- Piper runs as two sibling containers rather than host systemd units.
  The plan assumed Piper was already installed on the VPS; it is not,
  the host has no lingering user session to keep user units alive, and
  containers keep the TTS ports on an internal network unreachable from
  anywhere but Petal. One image, voice chosen per service, model cached
  in a shared volume -- so the pt-PT voice is a new service, not a new
  image.

- db.Backup + a `-backup` flag: VACUUM INTO, not a file copy. Petal runs
  in WAL mode, so the newest committed pages may live in petal.db-wal;
  copying the three files separately can capture a torn mid-checkpoint
  state. VACUUM INTO reads one coherent snapshot without taking a write
  lock, and emits a single file with no -wal/-shm companions. Refuses an
  existing destination so a failed run can't destroy the last good
  backup.

- deploy/backup-petal.sh: nightly snapshot, compress, push to millenia
  over headscale with a post-transfer size check, prune both sides.

- deploy/petal.env.example: LLM_TIMEOUT raised 30s -> 90s for the
  WAN+VPN round trip, since the voice and collocation passes send a
  whole document and the timeout is a hard deadline on Complete.

deploy/petal.env.example

@@ -0,0 +1,49 @@
+# Petal — production environment for the parodia.dev VPS.
+# Copy to the stack directory as `.env` (docker-compose.yml reads it via
+# env_file) and fill in the model names. Values the image already fixes
+# (PORT, DATABASE_PATH, IMAGE_DIR, TTS_CACHE_DIR, TTS endpoints) are set in
+# docker-compose.yml, not here.
+
+# --- Routing -----------------------------------------------------------------
+# Must match the DNS A record and the Traefik Host() rule.
+PETAL_HOST=petal.parodia.dev
+# Absolute origin the app knows itself by. Phase 16's OIDC redirect URI is
+# built from this, so it has to be the real public HTTPS origin.
+BASE_URL=https://petal.parodia.dev
+
+# The companion's bedtime nag and the night theme read the container clock.
+TZ=Europe/Lisbon
+
+# --- LLM (millenia, over headscale) ------------------------------------------
+# The only cross-VPN dependency. Petal degrades warmly when it's unreachable:
+# spell check, gloss, garden, search, export and read-aloud all keep working and
+# the status bar shows 小助手在休息 · Petal's helper is resting.
+#
+# 100.64.0.2 is millenia on the headscale network. vLLM must be bound to that
+# interface (NOT 0.0.0.0 — this host is public); see deploy/README.md.
+LLM_BACKEND=vllm
+LLM_ENDPOINT=http://100.64.0.2:8000
+LLM_MODEL=
+LLM_CHAT_MODEL=
+# 30s is the local-network default. Over WAN + VPN, with the voice and
+# collocation passes sending a whole document, that truncates real work — the
+# request is a hard deadline on Complete, and a timeout surfaces as the same
+# warm 502 as an unreachable model. 90s leaves headroom without letting a
+# genuinely wedged backend hang the pass forever.
+LLM_TIMEOUT=90s
+
+# --- Read-aloud (Piper sidecars) ---------------------------------------------
+# Endpoints are wired in docker-compose.yml; these pick the voice each sidecar
+# loads. Changing one means recreating that container so it downloads the model.
+TTS_VOICE_EN=en_US-amy-medium
+TTS_VOICE_ZH=zh_CN-huayan-medium
+TTS_AUDIO_FORMAT=mp3
+TTS_TIMEOUT=15s
+
+# --- Auth (Phase 16 — not wired yet) -----------------------------------------
+# Authentik already runs on this host. Filled in when the OIDC flow lands.
+# SESSION_SECRET=
+# AUTHENTIK_URL=https://auth.parodia.dev
+# AUTHENTIK_CLIENT_ID=petal
+# AUTHENTIK_CLIENT_SECRET=
+# PETAL_ALLOWED_SUBS=