Phase 4: triage actions (delete, move, mark; bulk select) (AUD-70)

Bulk select in the triage list, with delete/move/mark and a recoverable
delete (FR-6, FR-9).

- migration v4: messages.folder + messages.mailbox; folder/category views
- internal/mailops: app folder -> Gmail label moves over IMAP (IPv4), locate
  by X-GM-MSGID, create labels lazily, move/save/reply/restore, flag marks
- mailfetch: CREATE, UID MOVE (COPY+EXPUNGE fallback), STORE flags, X-GM-MSGID search
- server: POST /actions/bulk with CSRF, UI delete confirmation, second
  confirmation for messages classified Spam (never auto-deleted), per-message
  and summary action-log rows (FR-13)
- inbox: select-all toolbar, checkboxes, folder nav, Restore on non-inbox rows

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/store/store.go

@@ -123,6 +123,27 @@ func (s *Store) applyMigrations(ctx context.Context) error {
 			return fmt.Errorf("record schema version 3: %w", err)
 		}
 	}
+	if version < 4 {
+		// Phase 4 (FR-6/FR-9): triage actions file a message under an app folder
+		// and remember the Gmail label it currently lives in, so a delete can be
+		// undone by moving it back to the inbox.
+		for _, col := range []struct{ table, name, typ string }{
+			{"messages", "folder", "TEXT NOT NULL DEFAULT 'inbox'"},
+			{"messages", "mailbox", "TEXT NOT NULL DEFAULT 'INBOX'"},
+		} {
+			if err := s.ensureColumn(ctx, col.table, col.name, col.typ); err != nil {
+				return err
+			}
+		}
+		if _, err := s.db.ExecContext(ctx,
+			`CREATE INDEX IF NOT EXISTS idx_messages_folder ON messages (folder, received_at)`); err != nil {
+			return fmt.Errorf("index messages.folder: %w", err)
+		}
+		if _, err := s.db.ExecContext(ctx,
+			`INSERT OR IGNORE INTO schema_migrations (version) VALUES (4)`); err != nil {
+			return fmt.Errorf("record schema version 4: %w", err)
+		}
+	}
 	return nil
 }