Add sync conflict detection and resolution system

Implement conflict detection for concurrent reading progress updates from different devices. Adds conflict management endpoints for listing, viewing, and resolving conflicts.

- Add ConflictHandler with CRUD endpoints for conflict management
- Implement automatic conflict detection in KOReader progress updates
- Add WebSocket broadcast for real-time conflict notifications
- Add database query for listing user conflicts by status
- Add integration tests and Bruno API test collection

bruno/conflicts/List Conflicts.bru

@@ -0,0 +1,36 @@
+meta {
+  name: List Conflicts
+  type: http
+  seq: 1
+}
+
+get {
+  url: {{baseUrl}}/api/conflicts?status=unresolved
+  body: none
+  auth: bearer
+}
+
+headers: {
+  Authorization: Bearer {{token}}
+}
+
+docs: {
+  Lists all sync conflicts for the authenticated user.
+  
+  Query Parameters:
+  - status: Filter by resolution status (unresolved, user_resolved, auto_resolved, all)
+  
+  Response includes:
+  - conflicts: Array of conflict details
+  - total: Total number of conflicts
+  - unresolved: Number of unresolved conflicts
+  
+  Each conflict includes:
+  - id: Conflict UUID
+  - media_item_id: Associated book UUID
+  - media_item_title: Book title
+  - conflict_type: Type of conflict (progress, note, highlight)
+  - conflict_data: Side-by-side comparison of conflicting data
+  - resolution_status: Current status
+  - created_at: When conflict was detected
+}