[Backend][MVP] Implement auth, access control, and admin basics

[AlperKartkaya]

Goal

Implement the MVP auth flow and the minimum backend support needed for admin-only access.

Tasks

• Implement signup endpoint
• Implement login endpoint
• Implement email verification flow/endpoints
• Add password hashing
• Add token/session-based authentication for MVP
• Add basic role/access guard for admin-only routes
• Add simple admin endpoints for:
  • viewing users
  • viewing help requests and statuses
  • managing announcements
  • basic stats if simple to expose in MVP

Notes

Keep this practical.
Do not build a complex permission system for MVP.

Acceptance Criteria

• User can sign up and log in through backend APIs
• Email verification flow exists at backend level
• Protected routes can identify authenticated users
• Admin-only routes are guarded
• Minimal admin endpoints exist for MVP frontend needs

[delibas-rojhat]

Implemented in #101 and #102. All acceptance criteria met.