Add drag & drop reordering of trips on the dashboard

- trip_members.sort_order (per-user, migrated) drives GET /api/trips order
- PATCH /api/trips/:id/order updates only the caller's membership row
- Trip cards get a drag handle reusing the generalized row-reorder helper;
  clicking the card still navigates

src/server/db.js

@@ -23,6 +23,7 @@ CREATE TABLE IF NOT EXISTS trip_members (
   trip_id INTEGER NOT NULL REFERENCES trips(id),
   user_id INTEGER NOT NULL REFERENCES users(id),
   role TEXT NOT NULL DEFAULT 'editor',
+  sort_order INTEGER NOT NULL DEFAULT 0,
   PRIMARY KEY (trip_id, user_id)
 );
 
@@ -72,6 +73,7 @@ const MIGRATIONS = [
   { table: 'entries', column: 'rental', ddl: 'ALTER TABLE entries ADD COLUMN rental TEXT' },
   { table: 'entries', column: 'end_date', ddl: 'ALTER TABLE entries ADD COLUMN end_date TEXT' },
   { table: 'entries', column: 'transport_mode', ddl: 'ALTER TABLE entries ADD COLUMN transport_mode TEXT' },
+  { table: 'trip_members', column: 'sort_order', ddl: 'ALTER TABLE trip_members ADD COLUMN sort_order INTEGER NOT NULL DEFAULT 0' },
 ];
 
 function applyMigrations(db) {