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/Dismiss All Resolved.bru

@@ -0,0 +1,35 @@
+meta {
+  name: Dismiss All Resolved Conflicts
+  type: http
+  seq: 5
+}
+
+post {
+  url: {{baseUrl}}/api/conflicts/dismiss-all
+  body: none
+  auth: bearer
+}
+
+headers: {
+  Authorization: Bearer {{token}}
+}
+
+docs: {
+  Deletes all resolved conflicts for the authenticated user.
+  
+  Use this to:
+  - Clean up your conflicts list after reviewing resolutions
+  - Remove old resolved conflicts that are no longer needed
+  - Maintain a clean conflict history
+  
+  Response includes:
+  - deleted: Number of conflict records that were deleted
+  
+  Example response:
+  {
+    "deleted": 5
+  }
+  
+  Note: Only resolves conflicts with status "user_resolved"
+  are deleted. Unresolved conflicts are preserved.
+}