feat(db): VACUUM after retention and an Optimize button in Backup modal

SQLite DELETE marks pages free but doesn't shrink the file, so the
new retention job would keep DBs at their bloated size forever without
a follow-up VACUUM. Add db.vacuum() that runs PRAGMA-free VACUUM and
reports size_before/size_after/elapsed so callers can surface results.

The retention job now calls vacuum() automatically when it deleted at
least 1000 rows. Threshold avoids the multi-second VACUUM cost on quiet
days. Failure is logged, not raised — a missed VACUUM never crashes
the scheduler.

Power-user override: new "Optimize now" button in the Database Backup
modal triggers VACUUM on demand via POST /api/db/vacuum, alongside a
GET /api/db/size that drives the live "Current size" label. This way
users don't have to wait until 03:30 to reclaim space after the first
big retention pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/routes/api.py

@@ -4389,6 +4389,44 @@ def update_retention_settings_api():
         return jsonify({'success': False, 'error': str(e)}), 500
 
 
+@api_bp.route('/db/vacuum', methods=['POST'])
+def vacuum_database_api():
+    """Run SQLite VACUUM to reclaim space freed by DELETE statements.
+
+    Returned payload includes size_before / size_after / freed (bytes) and
+    elapsed_seconds so the UI can show how much space was reclaimed.
+    """
+    try:
+        db = _get_db()
+        if db is None:
+            return jsonify({'success': False, 'error': 'Database not available'}), 500
+        stats = db.vacuum()
+        logger.info(
+            f"Manual VACUUM: {stats['size_before']:,} -> {stats['size_after']:,} "
+            f"bytes (freed {stats['freed']:,} in {stats['elapsed_seconds']}s)"
+        )
+        return jsonify({'success': True, **stats})
+    except Exception as e:
+        logger.error(f"VACUUM failed: {e}", exc_info=True)
+        return jsonify({'success': False, 'error': str(e)}), 500
+
+
+@api_bp.route('/db/size', methods=['GET'])
+def get_database_size_api():
+    """Return current database file size in bytes (for the Optimize DB UI)."""
+    try:
+        db = _get_db()
+        if db is None:
+            return jsonify({'success': False, 'error': 'Database not available'}), 500
+        from pathlib import Path
+        path = Path(db.db_path)
+        size = path.stat().st_size if path.exists() else 0
+        return jsonify({'success': True, 'size': size, 'path': str(path)})
+    except Exception as e:
+        logger.error(f"Failed to read DB size: {e}")
+        return jsonify({'success': False, 'error': str(e)}), 500
+
+
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