chore: prepare standalone public repo

- Remove Mattermost integration (OAuth, channel bridge, file sync watcher,
  frontend import modal). RefBoard now ships as a self-contained app.
- Replace SSO Login screen with email/password form (+ optional register link
  gated by ALLOW_SELF_REGISTRATION).
- Add SEED_ADMIN_EMAIL / SEED_ADMIN_PASSWORD env-var bootstrap so a fresh
  install ships with an admin account on first boot (idempotent).
- ALLOW_SELF_REGISTRATION flag (default false) gates POST /api/auth/register.
  First user can always register (auto-promoted to admin).
- Drop mattermost_id and mm_file_id columns + board_channel_links table
  from the schema; remove related db helpers and exports.
- Add MIT LICENSE, comprehensive README, .env.example, docker-compose.yml
  (bundles MinIO so one command boots a working stack).
- Expand .gitignore for typical Node + Docker dev artefacts.

docker-compose.yml

@@ -0,0 +1,52 @@
+services:
+  minio:
+    image: minio/minio:latest
+    container_name: refboard-minio
+    command: server /data --console-address ":9001"
+    environment:
+      MINIO_ROOT_USER: ${MINIO_ACCESS_KEY:-minioadmin}
+      MINIO_ROOT_PASSWORD: ${MINIO_SECRET_KEY:-minioadmin}
+    ports:
+      - "9000:9000"   # S3 API
+      - "9001:9001"   # Web console
+    volumes:
+      - ./.docker-data/minio:/data
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:9000/minio/health/live"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+    restart: unless-stopped
+
+  refboard:
+    build: .
+    container_name: refboard
+    depends_on:
+      minio:
+        condition: service_healthy
+    environment:
+      PORT: 8000
+      NODE_ENV: ${NODE_ENV:-production}
+      JWT_SECRET: ${JWT_SECRET:?JWT_SECRET must be set in .env}
+      JWT_EXPIRES_IN: ${JWT_EXPIRES_IN:-7d}
+      DB_PATH: /app/data/refboard.db
+      MINIO_ENDPOINT: minio
+      MINIO_PORT: 9000
+      MINIO_USE_SSL: "false"
+      MINIO_ACCESS_KEY: ${MINIO_ACCESS_KEY:-minioadmin}
+      MINIO_SECRET_KEY: ${MINIO_SECRET_KEY:-minioadmin}
+      MINIO_BUCKET: ${MINIO_BUCKET:-refboard}
+      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