Add user auth with JWT, switch to bun test

- User table with company_id FK, unique email, role enum
- Register/login routes with bcrypt + JWT token generation
- Auth plugin with authenticate decorator and role guards
- Login uses globally unique email (no company header needed)
- Dev-auth plugin kept as fallback when JWT_SECRET not set
- Switched from vitest to bun:test (vitest had ESM resolution
  issues with zod in Bun's module structure)
- Upgraded to zod 4
- Added Dockerfile.dev and API service to docker-compose
- 8 tests passing (health + auth)

Dockerfile.dev

@@ -0,0 +1,18 @@
+FROM oven/bun:1
+
+WORKDIR /app
+
+# Install dependencies first (cached layer)
+COPY package.json bun.lock ./
+COPY packages/shared/package.json packages/shared/
+COPY packages/backend/package.json packages/backend/
+RUN bun install --frozen-lockfile
+
+# Copy source
+COPY . .
+
+WORKDIR /app/packages/backend
+
+EXPOSE 8000
+
+CMD ["bun", "--watch", "run", "src/main.ts"]