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/shared/src/constants.ts

@@ -0,0 +1,3 @@
+export const SUPPORTED_LANGUAGE_CODES = ["en", "it"] as const;
+
+export const SUPPORTED_POS = ["noun"] as const;