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.

.gitignore

@@ -1,6 +1,48 @@
+# Dependencies
 node_modules/
+.pnpm-store/
+
+# Build output
 dist/
+build/
+*.tsbuildinfo
+
+# Environment
 .env
+.env.local
+.env.*.local
+
+# Local data
+data/
+*.db
+*.db-journal
+*.db-wal
+*.db-shm
+
+# Logs
 *.log
-.worktrees/
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# Editor / OS
+.vscode/
+.idea/
+.DS_Store
+Thumbs.db
+
+# Test output
+coverage/
+.nyc_output/
+
+# Temp
+*.tmp
+.cache/
+
+# Internal tooling (not for public repo)
 .superpowers/
+.claude/
+docs/superpowers/
+
+# Docker volumes (local dev)
+.docker-data/