feat: add configurable status mappings for metadata providers

Add a status_mappings table to replace hardcoded provider status
normalization. Users can now configure how provider statuses (e.g.
"releasing", "finie") map to target statuses (e.g. "ongoing", "ended")
via the Settings > Integrations page.

- Migration 0038: status_mappings table with pre-seeded mappings
- Migration 0039: re-normalize existing series_metadata.status values
- API: CRUD endpoints for status mappings, DB-based normalize function
- API: new GET /series/provider-statuses endpoint
- Backoffice: StatusMappingsCard component with create target, assign,
  and delete capabilities
- Fix all clippy warnings across the API crate
- Fix missing OpenAPI schema refs (MetadataStats, ProviderCount)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/main.rs

@@ -137,6 +137,7 @@ async fn main() -> anyhow::Result<()> {
         .route("/series", get(books::list_all_series))
         .route("/series/ongoing", get(books::ongoing_series))
         .route("/series/statuses", get(books::series_statuses))
+        .route("/series/provider-statuses", get(books::provider_statuses))
         .route("/series/mark-read", axum::routing::post(reading_progress::mark_series_read))
         .route("/stats", get(stats::get_stats))
         .route("/search", get(search::search_books))