[Backend][MVP] Implement backend MVP aligned with agreed project scope

[AlperKartkaya]

Goal

Deliver the backend MVP in a simple, practical way that matches the agreed wiki scope and supports the frontend/mobile teams.

Scope

This issue tracks the backend work needed for MVP only.

Included MVP areas:

• auth flow
• profile, health, location, and privacy data
• help request lifecycle
• volunteer availability and assignment flow
• simple admin-only backend support for MVP screens

Not included:

• advanced matching algorithms
• real-time systems
• push notification infrastructure
• complex offline conflict-resolution engines
• major refactors or overengineered boundaries

Working principle

These areas are practical ownership guidelines, not strict architectural boundaries.

Shared setup should stay shared:

• src/config
• src/db
• src/routes
• small reusable helpers
• common response/error patterns

Sub-issues

• Auth, access control, and admin basics
• Profiles, privacy, health, and location
• Help request lifecycle
• Availability, matching, and assignment flow

Done criteria

• backend exposes MVP-ready endpoints for the four work areas
• status names and payloads are aligned with frontend/mobile expectations
• shared foundation stays simple and understandable
• no unnecessary abstraction is introduced before MVP