Add audit logging infrastructure (#6)

[Sakeeb91]

🔍 Feature: Audit Log & Activity Tracking

📌 Issue Reference

• Closes Title: Audit Log / Activity Tracking #6 – Audit Log / Activity Tracking

🧭 What’s Included

• Introduced AuditLog ORM model, Pydantic schemas, repository, and Alembic migration.
• Added AuditLogService, AuditTrailRecorder, and FastAPI router at /api/v1/audit-logs with list/read/create endpoints.
• Wired audit hooks into project, resource, maintenance-ticket, and document services using authenticated actor context.
• Enriched document service with download access logging and ensured API routers pass current_user metadata.
• Extended test infrastructure to support GeoAlchemy on SQLite and added targeted service/API regression tests.
• Authenticated helpers and documentation (backend/AUDIT_LOGGING.md) outlining usage, design, and testing strategy.

✅ Testing

• python3 -m pytest tests/services/test_audit_log_service.py -q
• python3 -m pytest tests/api/test_audit_logs.py -q
• python3 -m pytest tests/services/test_project_service_audit.py -q

📝 Notes for Reviewers

• GeoAlchemy2 callbacks are disabled in tests (see tests/conftest.py) so SQLite in-memory databases work without SpatiaLite.
• API routers now require current_user in request handlers to capture actor information—existing overrides were updated accordingly.