feat(auth): per-user UI language preference (sq default, en)

Add users.language ('sq'|'en', default 'sq'; migration 0003). Returned from
/api/auth/login and /api/auth/me (read from the DB, not the JWT — so changing it
needs no re-login). New PUT /api/auth/language for self-service. Loaded on login
and restored from any booth. Printed tickets stay Albanian (customer-facing).

packages/db/src/schema.ts

@@ -22,6 +22,12 @@ export const users = sqliteTable("users", {
   role: text("role", {
     enum: ["admin", "operator", "cashier", "readonly"],
   }).notNull(),
+  // Preferred UI language for this user (operator-facing). Loaded on login and
+  // restored from any booth. Albanian is the default. Printed tickets are NOT
+  // governed by this — they're always Albanian (customer-facing). See i18n.md.
+  language: text("language", { enum: ["sq", "en"] })
+    .notNull()
+    .default("sq"),
   createdAt: text("created_at")
     .notNull()
     .default(sql`(current_timestamp)`),