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

internal/database/queries/queries.sql

@@ -754,6 +754,13 @@ RETURNING *;
 -- name: DeleteSyncConflict :exec
 DELETE FROM sync_conflicts WHERE id = $1;
 
+-- name: ListAllConflictsByUserAndStatus :many
+SELECT sc.*, mi.title, mi.author
+FROM sync_conflicts sc
+JOIN media_items mi ON sc.media_item_id = mi.id
+WHERE sc.user_id = $1 AND sc.resolution_status = $2
+ORDER BY sc.created_at DESC;
+
 -- ============================================
 -- PHASE 3: KOREADER SYNC PROTOCOL (Weeks 7-9)
 -- ============================================