Add versioned DB migration system with automatic backup

migrations.py
- schema_migrations table tracks applied versions (version, description, applied_at)
- MIGRATIONS list is append-only; each entry is (version, description, sql)
- backup() dumps all user-data tables to a timestamped JSON file in backups/
  before any schema changes so data can be recovered if something goes wrong
- run_migrations() is idempotent: already-applied versions are skipped

Integration
- app.py calls _run_startup_migrations() at module load so every restart
  applies any pending migrations (no-op if schema is current)
- setup_routes.py calls run_migrations() after the initial setup form is
  submitted so all tables exist before the user hits the main page for the
  first time
- notes.py and admin.py: removed all per-request CREATE TABLE DDL; schema is
  now owned entirely by the migration system

Docker
- docker-compose.dev.yml: add backups-data volume so JSON backups survive
  container restarts and rebuilds
- Dockerfile: pre-create /app/backend/logs and /app/backend/backups so the
  directories exist even before volumes are mounted

Adding future schema changes
- Append a new (version, description, sql) tuple to MIGRATIONS in migrations.py
- The next restart will detect it as pending, back up first, then apply it

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app.py

@@ -93,6 +93,26 @@ def serve_frontend(path):
     return send_from_directory(dist_dir, 'index.html')
 
 
+def _run_startup_migrations():
+    """Run pending DB migrations if the database is already configured."""
+    if not is_configured():
+        logger.info("[startup] DB not yet configured — skipping migrations.")
+        return
+    try:
+        from util.db_pool import get_connection
+        from util.migrations import run_migrations
+        conn = get_connection()
+        try:
+            run_migrations(conn)
+        finally:
+            conn.close()
+    except Exception as e:
+        logger.error(f"[startup] Migration error: {e}")
+
+
+_run_startup_migrations()
+
+
 if __name__ == '__main__':
     os.makedirs("config", exist_ok=True)
     app.run(host='0.0.0.0', port=5000, debug=True)