refactor(onboarding): migrate to module-based structure and extract service layer

[Nirbhay21]

Summary

This PR restructures the backend from a layer-based architecture to a module-based architecture and extracts onboarding business logic into a dedicated service layer.
The onboarding flow is now clearly separated into:

• draft saving (PATCH /onboarding)
• completion (POST /onboarding/complete)
  Controllers are thin, business logic lives in services, and shared utilities are centralized.

Scope

• Migrate project from layer-based structure (controllers, routes, lib) to module-based architecture.
• Extract onboarding business logic into OnboardingService.
• Introduce dedicated completion endpoint for onboarding.
• Centralize shared logic into /shared.
• Improve schema typing and validation boundaries.

Architectural Changes

1. Module-Based Structure

Reorganized features into self-contained modules:

• modules/auth
• modules/health
• modules/onboarding

Each module now contains:

• controller
• routes
• schemas
• types
• service (if applicable)

Shared utilities moved to:

• shared/auth
• shared/constants
• shared/errors
• shared/middlewares

Environment config moved to:

• config/env.schema.ts

Onboarding Refactor

Controller → Service Extraction

• Removed fat onboarding.controller.ts
• Introduced OnboardingService (360 LOC)
• Controller now:
  • Handles HTTP orchestration only
  • Delegates all business logic to service layer

Endpoint Changes

• PUT /onboarding → replaced with:
  • PATCH /onboarding (save draft only)
  • POST /onboarding/complete (finalize onboarding)
    Completion logic separated from draft updates.

Validation Changes

• Removed old onboarding-status.schema.ts
• Introduced onboarding-status-update.schema.ts
• Enforced:
  • PATCH cannot mark onboarding as completed
  • Completion must go through /onboarding/complete

Improved DB draft typing:

CandidateOnboardingDraftData | RecruiterOnboardingDraftData | null