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/setup-env.ts

@@ -0,0 +1,7 @@
+// bun test preload — replicates the env block from the old vitest.config.ts.
+// Runs before any test module imports, so the config singleton (which reads
+// process.env at import time) gets the same test values it had under vitest.
+process.env.DISCORD_TOKEN = "test-discord-token";
+process.env.DATABASE_URL = "postgres://localhost:6432/test";
+process.env.AI_ANALYSIS_ENABLED = "true";
+process.env.AI_LLM_API_KEY = "sk-test";