feat: zero-config first boot and pluggable storage backend

Two changes that drop the friction in self-hosting RefBoard so the install
story becomes "docker compose up, open the URL".

1. JWT_SECRET is now optional. On first boot the backend generates a
   64-byte random secret and persists it in the existing settings table.
   process.env.JWT_SECRET still wins when set, so ops setups that manage
   secrets out-of-band are unaffected. The prod-throws-without-env guard
   is gone (auto-generation is a strictly safer default than the previous
   hardcoded dev fallback).

2. STORAGE_BACKEND=fs|minio picks between MinIO (default, unchanged) and
   a new local-filesystem adapter. The FS adapter exposes a fake minioClient
   that mirrors the methods RefBoard calls (statObject, getObject,
   getPartialObject, listObjectsV2, putObject, removeObject(s), bucketExists,
   makeBucket), so consumers swap require('./minio') for require('./storage')
   and nothing else changes. Sidecar .mime files hold Content-Type so the
   range-aware media proxy still serves the right response headers.

examples/compose/minimal-fs.yml is the single-container variant that uses
the FS adapter. The default docker-compose.yml still spins up MinIO.

README quick-start collapses to one block (cp .env, docker compose pull,
docker compose up -d). The first user you register on the Login screen is
auto-promoted to admin, same as before.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

examples/compose/minimal-fs.yml

@@ -0,0 +1,39 @@
+# Single-container RefBoard with local-filesystem storage.
+#
+# Use this instead of the repo-root docker-compose.yml when you don't want to
+# run MinIO. State is stored under ./.docker-data/refboard/ (SQLite + media
+# bytes + JWT secret, all together — back this directory up).
+#
+# Usage:
+#   cp .env.example .env       # most values can stay at defaults
+#   docker compose -f examples/compose/minimal-fs.yml up -d
+
+services:
+  refboard:
+    image: ${REFBOARD_IMAGE:-ghcr.io/metalfinger/refboard:latest}
+    container_name: refboard
+    environment:
+      PORT: 8000
+      NODE_ENV: ${NODE_ENV:-production}
+      # JWT_SECRET is optional — RefBoard generates one on first boot and
+      # persists it in the SQLite settings table. Override here if you'd
+      # rather manage it out-of-band.
+      JWT_SECRET: ${JWT_SECRET:-}
+      JWT_EXPIRES_IN: ${JWT_EXPIRES_IN:-7d}
+      DB_PATH: /app/data/refboard.db
+      STORAGE_BACKEND: fs
+      STORAGE_DATA_DIR: /app/data/storage
+      PUBLIC_URL: ${PUBLIC_URL:-}
+      MAX_FILE_SIZE_MB: ${MAX_FILE_SIZE_MB:-200}
+      CORS_ORIGIN: ${CORS_ORIGIN:-*}
+      SEED_ADMIN_EMAIL: ${SEED_ADMIN_EMAIL:-}
+      SEED_ADMIN_PASSWORD: ${SEED_ADMIN_PASSWORD:-}
+      SEED_ADMIN_USERNAME: ${SEED_ADMIN_USERNAME:-}
+      SEED_ADMIN_DISPLAY_NAME: ${SEED_ADMIN_DISPLAY_NAME:-}
+      ALLOW_SELF_REGISTRATION: ${ALLOW_SELF_REGISTRATION:-false}
+      REFBOARD_API_KEY: ${REFBOARD_API_KEY:-}
+    ports:
+      - "8000:8000"
+    volumes:
+      - ./.docker-data/refboard:/app/data
+    restart: unless-stopped