feat(db): add drizzle schema for vocabulary and deck tables

- terms, translations, term_glosses with cascade deletes and pos check constraint
- language_pairs with source/target language check constraints and no-self-pair guard
- users with openauth_sub as identity provider key
- decks and deck_terms with composite PK and position ordering
- indexes on all hot query paths (distractor generation, deck lookups, FK joins)
- SUPPORTED_POS and SUPPORTED_LANGUAGE_CODES as single source of truth in @glossa/shared

packages/db/package.json

@@ -7,6 +7,7 @@
     "build": "tsc"
   },
   "dependencies": {
+    "@glossa/shared": "workspace:*",
     "dotenv": "^17.3.1",
     "drizzle-orm": "^0.45.1",
     "pg": "^8.20.0"