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/web/package.json

@@ -13,6 +13,7 @@
     "@tailwindcss/vite": "^4.2.2",
     "@tanstack/react-router": "^1.168.1",
     "@tanstack/react-router-devtools": "^1.166.10",
+    "better-auth": "^1.6.2",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
     "tailwindcss": "^4.2.2"