feat(backend): Add AuditLog database schema and migration

Implement AuditLog entity and EF Core configuration for Sprint 2 Story 1 Task 1.

Changes:
- Created AuditLog entity with multi-tenant support
- Added EF Core configuration with JSONB columns for PostgreSQL
- Created composite indexes for query optimization
- Generated database migration (20251104220842_AddAuditLogTable)
- Updated PMDbContext with AuditLog DbSet and query filter
- Updated task status to in_progress in sprint plan

Technical Details:
- PostgreSQL JSONB type for OldValues/NewValues (flexible schema)
- Composite index on (TenantId, EntityType, EntityId) for entity history queries
- Timestamp index (DESC) for recent logs queries
- UserId index for user activity tracking
- Multi-tenant query filter applied to AuditLog

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/plans/sprint_2_story_1_task_1.md

@@ -1,9 +1,10 @@
 ---
 task_id: sprint_2_story_1_task_1
 story: sprint_2_story_1
-status: not_started
+status: in_progress
 estimated_hours: 4
 created_date: 2025-11-05
+start_date: 2025-11-05
 assignee: Backend Team
 ---