Separate face detection into standalone daemon

- Strip all face code from update.rb; add shared log helper writing to
  /opt/albumen/log/albumen.log with [update] prefix.  update.rb now owns
  only album.json; face_daemon.rb owns faces.json.
- New scripts/face_daemon.rb: polls MEDIA_ROOT for unprocessed images,
  calls faces.py in batches, writes per-directory faces.json sidecars
  atomically.  Graceful SIGTERM/SIGINT shutdown between directories.
- New config/face_daemon.service: systemd unit running as albumen user,
  Restart=on-failure, logs via SyslogIdentifier=albumen-faces.
- app.rb: add FACES_ENABLED constant; load_faces() helper reads faces.json;
  album_files() merges face data into each entry as :faces field.
- Update README.md and DESIGN.md to document the new daemon architecture,
  faces.json schema, and service management commands.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/config/face_daemon.service b/config/face_daemon.service
new file mode 100644
index 0000000..4babccc
--- /dev/null
+++ b/config/face_daemon.service
@@ -0,0 +1,27 @@
+[Unit]
+Description=Albumen face detection daemon
+After=network.target albumen.service
+Wants=albumen.service
+
+[Service]
+Type=simple
+User=albumen
+Group=albumen
+WorkingDirectory=/opt/albumen
+
+Environment=MEDIA_ROOT=/var/albumen
+Environment=CONFIG_PATH=/opt/albumen/config.yml
+Environment=LOG_PATH=/opt/albumen/log/albumen.log
+Environment=VENV_PYTHON=/opt/albumen/venv/bin/python3
+Environment=FACES_SCRIPT=/opt/albumen/scripts/faces.py
+
+ExecStart=/usr/bin/ruby /opt/albumen/scripts/face_daemon.rb
+Restart=on-failure
+RestartSec=30
+
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=albumen-faces
+
+[Install]
+WantedBy=multi-user.target