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/src/lib/auth.ts

@@ -0,0 +1,17 @@
+import { betterAuth } from "better-auth";
+import { drizzleAdapter } from "better-auth/adapters/drizzle";
+import { db } from "@glossa/db";
+
+export const auth = betterAuth({
+  database: drizzleAdapter(db, { provider: "pg" }),
+  socialProviders: {
+    google: {
+      clientId: process.env["GOOGLE_CLIENT_ID"] as string,
+      clientSecret: process.env["GOOGLE_CLIENT_SECRET"] as string,
+    },
+    github: {
+      clientId: process.env["GITHUB_CLIENT_ID"] as string,
+      clientSecret: process.env["GITHUB_CLIENT_SECRET"] as string,
+    },
+  },
+});