V134: folder-header customization columns on phoenix_kit_media_folders.

Adds the fields backing the Media browser's folder hero header / "Edit header" panel:

• cover_file_uuid UUID — background image (a normal uploaded file living in the folder, excluded from its visible listing). FK to phoenix_kit_files with ON DELETE SET NULL so deleting the file self-heals the reference.
• logo_file_uuid UUID — icon/logo shown in the header (same FK).
• header_size TEXT NOT NULL DEFAULT 'medium' — header height: small / medium / large.
• header_show_title, header_show_icon, header_show_creator, header_show_date, header_show_file_count, header_show_description, header_show_background BOOLEAN — per-folder visibility toggles for each header element (default on). Creator / date / file-count are independent so users can show just the pieces they want.

All nullable / defaulted, so existing folders render the header as before. Idempotent: ADD COLUMN IF NOT EXISTS plus guarded ADD CONSTRAINT, safe to re-run.