Enhance Security, Monitoring, and Dependency Hygiene

**Goal**: Strengthen production readiness with auditing, logging, validation, and basic rate limiting.

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### Tasks
- Add `npm audit` to CI for vulnerability checks
- Set up Dependabot for automated dependency updates
- Build logging utilities (see `/src/lib/logger.ts` example)
- Validate required environment variables on startup
- Implement simple rate limiting middleware in `/src/middleware.ts`

**References:** See [QUICK_WINS.md](https://github.com/adhit-r/aran/blob/main/QUICK_WINS.md)

**Acceptance Criteria**
- CI fails on high vulnerabilities
- Automatic PRs for outdated dependencies
- New logger and env validation modules
- Rate limiting at entry for app endpoints

Start with QUICK_WINS.md, then inspect the existing CI configuration and the entry points in /src/lib/logger.ts and /src/middleware.ts. Separate the audit, Dependabot, logging, environment validation, and rate-limiting tasks before changing anything. Done means high-severity vulnerabilities fail CI, dependency update PRs are enabled, logging and startup validation exist, and app endpoints receive rate limiting.

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