feat: implement volunteer availability and assignment matching

[berat-sayin]

Description

This PR implements the backend infrastructure for the Volunteer Availability module and the Matching/Assignment flow for the MVP. It
allows volunteers to declare their availability and automatically pairs them with pending help requests based on matching criteria.

Key Features

• Availability Toggle: New endpoint to turn availability on/off with optional GPS coordinate updates.
• Offline Sync: Support for syncing a batch of availability records from mobile devices that were previously offline.
• Automated Matching: Simple deterministic logic that matches an available volunteer with the oldest pending help_request that fits their
  need_types.
• Assignment Management: Volunteers can view their current assignment, mark it as resolved, or cancel it.
• Re-evaluation Flow: If an assignment is cancelled, the request is put back into the PENDING pool, and the volunteer is immediately
  searched for a new matching task.

Technical Changes

• Module Structure: Created repository.js, service.js, controller.js, and validators.js within backend/src/modules/availability.
• Database Integration: Utilizes volunteers, availability_records, and assignments tables.
• Routing: Mounted the availabilityRouter in the main API router under /api/availability.
• Security: All endpoints are protected by the existing requireAuth middleware.

New Endpoints

• POST /api/availability/toggle: Update current availability.
• POST /api/availability/sync: Sync offline history.
• GET /api/availability/my-assignment: View active task.
• POST /api/availability/assignments/:id/cancel: Drop current task.
• POST /api/availability/assignments/resolve: Mark task as completed.

Related to the issue #90 and its sub-issues #113, #114, #116, #118, #119.

[AlperKartkaya]

Seems quite alright, if anything goes wrong we can check later on.