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

pnpm-lock.yaml

@@ -112,6 +112,9 @@ importers:
 
   packages/db:
     dependencies:
+      '@glossa/shared':
+        specifier: workspace:*
+        version: link:../shared
       dotenv:
         specifier: ^17.3.1
         version: 17.3.1