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/migrations_sqlite/20260319000001_multi_provider_instances.sql

@@ -0,0 +1,17 @@
+-- Recreate provider_configs with per-instance id as PK
+CREATE TABLE provider_configs_new (
+  id            TEXT PRIMARY KEY,
+  provider_type TEXT NOT NULL,
+  config_json   TEXT NOT NULL,
+  enabled       INTEGER NOT NULL DEFAULT 1,
+  updated_at    TEXT NOT NULL
+);
+INSERT INTO provider_configs_new (id, provider_type, config_json, enabled, updated_at)
+  SELECT provider_type, provider_type, config_json, enabled, updated_at
+  FROM provider_configs;
+DROP TABLE provider_configs;
+ALTER TABLE provider_configs_new RENAME TO provider_configs;
+
+-- Scope local_files_index entries by provider instance
+ALTER TABLE local_files_index ADD COLUMN provider_id TEXT NOT NULL DEFAULT 'local';
+CREATE INDEX IF NOT EXISTS idx_local_files_provider ON local_files_index(provider_id);