feat: multi-instance provider support

- provider_configs: add id TEXT PK; migrate existing rows (provider_type becomes id)
- local_files_index: add provider_id column + index; scope all queries per instance
- ProviderConfigRow: add id field; add get_by_id to trait
- LocalIndex:🆕 add provider_id param; all SQL scoped by provider_id
- factory: thread provider_id through build_local_files_bundle
- AppState.local_index: Option<Arc<LocalIndex>> → HashMap<String, Arc<LocalIndex>>
- admin_providers: restructured routes (POST /admin/providers create, PUT/DELETE /{id}, POST /test)
- admin_providers: use row.id as registry key for jellyfin and local_files
- files.rescan: optional ?provider=<id> query param
- frontend: add id to ProviderConfig, update api/hooks, new multi-instance panel UX

k-tv-backend/api/src/main.rs

@@ -99,8 +99,8 @@ async fn main() -> anyhow::Result<()> {
     .await?;
 
     #[cfg(feature = "local-files")]
-    if let Some(idx) = bundle.local_index {
-        *state.local_index.write().await = Some(idx);
+    if !bundle.local_index.is_empty() {
+        *state.local_index.write().await = bundle.local_index;
     }
     #[cfg(feature = "local-files")]
     if let Some(tm) = bundle.transcode_manager {