feat(settings): add database panel with info, backup and import

Add a Database section under Settings (split into Integrations/System
tabs) showing SQLite file size, last-modified date, a proportional
table-usage bar and per-table row counts. Supports downloading a
consistent backup and importing a .db file that overwrites the entire
database, with an explicit overwrite warning and confirmation.

Backend adds GET /api/database/info, GET /api/database/backup and
POST /api/database/import; DB_FILE is now exported from server-db.

server-db.ts

@@ -1,7 +1,7 @@
 import Database from 'better-sqlite3';
 import path from 'path';
 
-const DB_FILE = path.join(process.cwd(), 'ghostgrid.db');
+export const DB_FILE = path.join(process.cwd(), 'ghostgrid.db');
 
 console.log(`[Database] Connecting to SQLite database at: ${DB_FILE}`);
 const db = new Database(DB_FILE);