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>

.github/workflows/deploy.yml

@@ -32,29 +32,26 @@ jobs:
         with:
           node-version: 22
 
-      - name: Install pnpm
-        run: corepack enable && corepack prepare pnpm@11 --activate
+      - name: Setup Bun
+        uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: 1.3.14
 
-      - name: Install deps (backend)
-        working-directory: services/backend
-        run: pnpm install --ignore-scripts --no-frozen-lockfile
-
-      - name: Typecheck + test (backend)
+      - name: Install deps + test (backend)
         working-directory: services/backend
         run: |
+          bun install --frozen-lockfile
           ./node_modules/.bin/tsc --noEmit
-          # e2e.test.ts requires a live backend (API_BASE) — run unit tests only
-          ./node_modules/.bin/vitest run --exclude "src/e2e.test.ts"
+          # src/e2e.test.ts requires a live backend (API_BASE) — unit tests
+          # live in tests/ and are excluded by the bun test dir.
+          bun test tests/
 
-      - name: Install deps (discord-gateway)
-        working-directory: services/discord-gateway
-        run: pnpm install --ignore-scripts --no-frozen-lockfile
-
-      - name: Typecheck + test (discord-gateway)
+      - name: Install deps + test (discord-gateway)
         working-directory: services/discord-gateway
         run: |
+          bun install --frozen-lockfile
           ./node_modules/.bin/tsc --noEmit
-          ./node_modules/.bin/vitest run
+          bun test tests/
 
       - name: Biome check (all services)
         run: |