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/src/lib/prisma.ts

@@ -1,4 +1,3 @@
-import 'dotenv/config';
 import { type Prisma, PrismaClient } from '@generated/prisma/client';
 import type { Result } from '@gruperly/shared';
 import { PrismaPg } from '@prisma/adapter-pg';