feat(core+frontend): add P2P sync infrastructure with WebSocket support

- **Core Updates:**
  - Implemented `P2pSyncService` interface with platform-specific WebSocket implementations (`JvmP2pSyncService` and no-op for JS).
  - Developed `SyncEvent` sealed class hierarchy to handle peer synchronization events (e.g., `PingEvent`, `PongEvent`, `DataChangedEvent`, etc.).

- **Frontend Integration:**
  - Introduced `SyncManager` to manage peer discovery and synchronization, coupled with `NetworkDiscoveryService`.
  - Updated dependency injection to include `syncModule` for platform-specific sync service initialization.
  - Enhanced `BewerbViewModel` to support new sync capabilities, including observing sync events and UI updates for connected peers.

- **Backend Enhancements:**
  - Added ZNS-specific fields (`zns_nummer`, `zns_abteilung`) to Bewerb table for idempotent imports.
  - Introduced import ZNS logic to handle duplicates and align with SyncManager updates.

- **UI Improvements:**
  - Enhanced `TurnierBewerbeTab` with updated dialogs (ZNS imports, sync status) and dynamic previews.
  - Improved network syncing feedback and error handling in frontend components.

- **DB Changes:**
  - Added migration for new column fields in the Bewerb table with relevant indexing for ZNS import optimizations.

frontend/shells/meldestelle-desktop/src/jvmMain/kotlin/at/mocode/desktop/screens/preview/ScreenPreviews.kt

@@ -128,7 +128,12 @@ fun PreviewTurnierBewerbeTab() {
     override suspend fun generate(bewerbId: Long): Result<List<StartlistenZeile>> = Result.success(emptyList())
     override suspend fun getByBewerb(bewerbId: Long): Result<List<StartlistenZeile>> = Result.success(emptyList())
   }
-  val vm = BewerbViewModel(mockRepo, mockStartlistenRepo, null, 1L)
+  val vm = BewerbViewModel(
+    repo = mockRepo,
+    startlistenRepo = mockStartlistenRepo,
+    syncManager = null,
+    turnierId = 1L
+  )
   MaterialTheme {
     BewerbeTabContent(viewModel = vm, turnierId = 1L)
   }