Add rider/driver role selection after sign-up

- users.role column (+ migration in seed script)
- /(api)/user: GET role by clerkId, PATCH to set role
- Role selection screen; drivers get placeholder driver-home
- Root index routes by role; hardened location handling on home

scripts/seed-db.mjs

@@ -26,9 +26,13 @@ await sql`CREATE TABLE IF NOT EXISTS users (
   name VARCHAR(255) NOT NULL,
   email VARCHAR(255) NOT NULL,
   clerk_id VARCHAR(255) NOT NULL,
+  role VARCHAR(20),
   created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
 )`;
 
+// For databases created before roles existed.
+await sql`ALTER TABLE users ADD COLUMN IF NOT EXISTS role VARCHAR(20)`;
+
 await sql`CREATE TABLE IF NOT EXISTS drivers (
   id SERIAL PRIMARY KEY,
   first_name VARCHAR(100) NOT NULL,