chore: migrate monorepo toolchain pnpm+vitest → bun (bun test, bunfig preload, bun.lock)

- Gateway + backend + frontend: pnpm/vitest fully removed → bun 1.3.14
  (bun install, bun test tests/, bunfig.toml [test] preload, bun.lock).
- vitest configs deleted; vitest→bun facade (jest/mock/spyOn/waitForCompat)
  keeps the vitest-style assertions working under bun:test.
- flake.nix: bunInstall switch; pruned prod-pass now removes post-pnpm
  dev-toolchain trees; frontend builds Next standalone via bun's next.
- CI: deploy.yml installs with bun + runs bun test tests/ per service.
- 190 tests green (146 gateway + 37 backend pass, 14 skip), 0 fail;
  tsc + biome across all 3 services clean.

Co-authored-by: git-migration[bot] <noreply@gmw.local>

services/discord-gateway/tests/cacheBatchLookup.test.ts

@@ -6,12 +6,12 @@
 // single-key getter: unexpired rows only, malformed rows skipped, verdicts
 // normalized through the shared parser. The DB layer is mocked — no live
 // Postgres in unit tests.
-import { beforeEach, describe, expect, it, vi } from "vitest";
+import { beforeEach, describe, expect, it, jest, mock } from "bun:test";
 
-const executeAll = vi.fn();
-const executeGet = vi.fn();
+const executeAll = jest.fn();
+const executeGet = jest.fn();
 
-vi.mock("../src/shared/database/drizzle.js", () => ({
+mock.module("../src/shared/database/drizzle.js", () => ({
   executeAll: (...args: unknown[]) => executeAll(...args),
   executeGet: (...args: unknown[]) => executeGet(...args),
 }));