refactor: migrate monorepo from pnpm to Bun runtime and workspaces

- root: bun workspaces via package.json, packageManager bun@1.4.0, scripts use bun --filter
- backend: Bun.serve, bun test (vitest removed), bun --watch dev script, tsconfig types bun
- tests: migrate 6 test files from vitest to bun:test (mock.module)
- validate: replace @hono/zod-validator with hono validator + zod safeParse (fixes TS2589)
- lockfile: bunfig.toml saveTextLockfile, regenerated bun.lock (text) for turbo
- docs: AGENTS.md, README.md, stack.md updated to Bun stack

apps/backend/tsconfig.json

@@ -2,7 +2,7 @@
   "extends": "@gruperly/config/tsconfig.base.json",
   "compilerOptions": {
     "moduleResolution": "bundler",
-    "types": ["node"],
+    "types": ["bun"],
     "declaration": false,
     "noEmit": false,
     "outDir": "./dist",