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/Get Conflict Details.bru

@@ -0,0 +1,58 @@
+meta {
+  name: Get Conflict Details
+  type: http
+  seq: 2
+}
+
+get {
+  url: {{baseUrl}}/api/conflicts/{{conflict_id}}
+  body: none
+  auth: bearer
+}
+
+headers: {
+  Authorization: Bearer {{token}}
+}
+
+docs: {
+  Retrieves detailed information about a specific conflict.
+  
+  Path Parameters:
+  - conflict_id: UUID of the conflict
+  
+  Response includes:
+  - id: Conflict UUID
+  - media_item_id: Associated book UUID
+  - media_item_title: Book title
+  - conflict_type: Type of conflict
+  - conflict_data: Side-by-side comparison with sources:
+    * source: Device/source identifier (koreader, kobo, web, etc.)
+    * timestamp: When this progress was recorded
+    * data: The conflicting data (percentage, epubcfi, chapter, etc.)
+  - resolution_status: Current status
+  - resolution_data: If resolved, includes resolution details
+  - resolved_by: User ID who resolved it (if applicable)
+  - resolved_at: When it was resolved (if applicable)
+  - created_at: When conflict was detected
+  
+  Example conflict_data:
+  {
+    "koreader": {
+      "source": "koreader",
+      "timestamp": "2026-01-30T20:10:00Z",
+      "data": {
+        "percentage": 0.45,
+        "epubcfi": "epubcfi(/6/4/2:15)",
+        "chapter": 3
+      }
+    },
+    "kobo": {
+      "source": "kobo",
+      "timestamp": "2026-01-30T20:05:00Z",
+      "data": {
+        "percentage": 0.42,
+        "location": "unknown"
+      }
+    }
+  }
+}