feat(api): implement game terms query with double join

- Add double join on translations for source/target languages
- Left join term_glosses for optional source-language glosses
- Filter difficulty on target side only (intentionally asymmetric:
  a word's difficulty can differ between languages, and what matters
  is the difficulty of the word being learned)
- Return neutral field names (sourceText, targetText, sourceGloss)
  instead of quiz semantics; service layer maps to prompt/answer
- Tighten term_glosses unique constraint to (term_id, language_code)
  to prevent the left join from multiplying question rows
- Add TODO for ORDER BY RANDOM() scaling post-MVP

packages/db/src/db/schema.ts

@@ -51,11 +51,7 @@ export const term_glosses = pgTable(
     created_at: timestamp({ withTimezone: true }).defaultNow().notNull(),
   },
   (table) => [
-    unique("unique_term_gloss").on(
-      table.term_id,
-      table.language_code,
-      table.text,
-    ),
+    unique("unique_term_gloss").on(table.term_id, table.language_code),
     check(
       "language_code_check",
       sql`${table.language_code} IN (${sql.raw(SUPPORTED_LANGUAGE_CODES.map((l) => `'${l}'`).join(", "))})`,