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.

.env.example

@@ -0,0 +1,58 @@
+# ────────────────────────────────────────────────
+# RefBoard — Environment Configuration
+# Copy this file to `.env` and edit values for your install.
+# ────────────────────────────────────────────────
+
+# ---- Server ----
+PORT=8000
+NODE_ENV=development
+CORS_ORIGIN=*
+
+# ---- JWT auth ----
+# REQUIRED in production. Generate with: openssl rand -base64 64
+JWT_SECRET=change-me-to-a-long-random-string
+JWT_EXPIRES_IN=7d
+
+# ---- SQLite database ----
+# Path inside the running process. Defaults to /app/data/refboard.db (Docker).
+# For local non-Docker dev, set this to ./data/refboard.db (relative to backend/).
+DB_PATH=/app/data/refboard.db
+
+# ---- Object storage (MinIO / any S3-compatible) ----
+MINIO_ENDPOINT=minio
+MINIO_PORT=9000
+MINIO_USE_SSL=false
+MINIO_ACCESS_KEY=minioadmin
+MINIO_SECRET_KEY=minioadmin
+MINIO_BUCKET=refboard
+
+# Public-facing base URL for media. Leave empty to serve via the backend proxy
+# (recommended for self-hosted single-box installs).
+PUBLIC_URL=
+
+# Max upload size in MB (per file). Defaults to 200.
+MAX_FILE_SIZE_MB=200
+
+# ---- First-run admin bootstrap ----
+# When the server starts and no user with this email exists, RefBoard will
+# create one with role=admin. Idempotent — runs every boot but only seeds once.
+# Leave blank to skip seeding.
+SEED_ADMIN_EMAIL=
+SEED_ADMIN_PASSWORD=
+SEED_ADMIN_USERNAME=
+SEED_ADMIN_DISPLAY_NAME=
+
+# ---- Self-registration ----
+# When "true", the public /api/auth/register endpoint accepts new signups.
+# When "false" (default), only admins can create accounts (via /api/admin/users).
+# The very first user can always register, regardless of this flag, and is
+# auto-promoted to admin.
+ALLOW_SELF_REGISTRATION=false
+
+# Frontend mirror — controls whether the Login screen shows the "Register" link.
+# Must be set at *build* time (Vite) for the bundled frontend, not at runtime.
+VITE_ALLOW_SELF_REGISTRATION=false
+
+# ---- Optional: API key for bot / programmatic upload ----
+# If set, /api/upload/api-key endpoints require this header value.
+REFBOARD_API_KEY=