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/Delete Conflict.bru

@@ -0,0 +1,33 @@
+meta {
+  name: Delete Conflict
+  type: http
+  seq: 4
+}
+
+delete {
+  url: {{baseUrl}}/api/conflicts/{{conflict_id}}
+  body: none
+  auth: bearer
+}
+
+headers: {
+  Authorization: Bearer {{token}}
+}
+
+docs: {
+  Deletes a specific conflict record.
+  
+  Path Parameters:
+  - conflict_id: UUID of the conflict to delete
+  
+  Use this when:
+  - A conflict was created in error
+  - You want to dismiss a conflict without resolving it
+  - The conflict is no longer relevant
+  
+  Response: 204 No Content on success
+  
+  Note: This permanently removes the conflict record.
+  Consider resolving the conflict instead if you want to
+  maintain an audit trail of what happened.
+}