feat(project): add Docker Compose with flexible storage and DB modes

Bundle the app + Postgres into a compose stack on top of the existing image.

- app: builds the image, publishes ${APP_PORT:-42776}, reads .env, pins
  STATIC_DIR so SPA serving can't be disabled by an empty value
- db: postgres:14-alpine under the "with-db" profile; toggle it off via
  COMPOSE_PROFILES to point the app at a Postgres on the host instead
  (host.docker.internal), with depends_on required:false so it stays optional

Storage and the DB data dir each default to a named volume but can be bind
mounted to a host folder via FILES_DIR / THUMBS_DIR / IMPORT_DIR / DB_DIR.
Add PUID/PGID (via user:) so bind-mounted folders are writable by the
non-root container.

Run the container as a dedicated non-root user "tanabata" with uid/gid 42776,
reusing the project's signature number (also the default port). Document every
variable in .env.example.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.dockerignore

@@ -6,6 +6,10 @@
 .gitignore
 **/.DS_Store
 
+# Compose file — used to build/run, not needed inside the image context
+docker-compose.yml
+docker-compose.*.yml
+
 # Secrets and local env
 .env
 .env.*