feat(api): integrate Better Auth with Drizzle adapter and social providers

- Add Better Auth config with Google + GitHub social providers
- Mount auth handler on /api/auth/* in Express
- Generate and migrate auth tables (user, session, account, verification)
- Deduplicate term_glosses data for tighter unique constraint
- Drop legacy users table

apps/api/package.json

@@ -12,6 +12,7 @@
   "dependencies": {
     "@glossa/db": "workspace:*",
     "@glossa/shared": "workspace:*",
+    "better-auth": "^1.6.2",
     "express": "^5.2.1"
   },
   "devDependencies": {