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

packages/db/drizzle/meta/_journal.json

@@ -29,6 +29,13 @@
       "when": 1775513042249,
       "tag": "0003_greedy_revanche",
       "breakpoints": true
+    },
+    {
+      "idx": 4,
+      "version": "7",
+      "when": 1775986238669,
+      "tag": "0004_red_annihilus",
+      "breakpoints": true
     }
   ]
-}
+}