feat: add scheduled scans (cron-like recurring scans)

- New `scheduled_scans` table with daily/weekly/monthly frequencies
- asyncio background scheduler loop checks for due schedules every 60s
- 6 REST endpoints: CRUD + toggle enabled + run-now
- `scheduled_scan_id` FK added to scans table; migrated automatically
- Frontend: Schedules page (list + create form), Schedules nav link,
  "Scheduled" badge on ScanDetails when scan was triggered by a schedule

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

flight-comparator/database/init_db.py

@@ -167,6 +167,29 @@ def _migrate_add_routes_unique_index(conn, verbose=True):
         print("   ✅ Migration complete: uq_routes_scan_dest index created")
 
 
+def _migrate_add_scheduled_scan_id_to_scans(conn, verbose=True):
+    """
+    Migration: add scheduled_scan_id column to scans table.
+
+    Existing rows get NULL (manual scans). New column has no inline FK
+    declaration because SQLite's ALTER TABLE ADD COLUMN doesn't support it;
+    the relationship is enforced at the application level.
+    """
+    cursor = conn.execute("PRAGMA table_info(scans)")
+    columns = [row[1] for row in cursor.fetchall()]
+    if 'scheduled_scan_id' in columns:
+        return  # Already migrated
+
+    if verbose:
+        print("   🔄 Migrating scans table: adding scheduled_scan_id column...")
+
+    conn.execute("ALTER TABLE scans ADD COLUMN scheduled_scan_id INTEGER")
+    conn.commit()
+
+    if verbose:
+        print("   ✅ Migration complete: scheduled_scan_id column added to scans")
+
+
 def initialize_database(db_path=None, verbose=True):
     """
     Initialize or migrate the database.
@@ -212,6 +235,7 @@ def initialize_database(db_path=None, verbose=True):
     # Apply migrations before running schema
     _migrate_relax_country_constraint(conn, verbose)
     _migrate_add_routes_unique_index(conn, verbose)
+    _migrate_add_scheduled_scan_id_to_scans(conn, verbose)
 
     # Load and execute schema
     schema_sql = load_schema()