feat: add metadata refresh job to re-download metadata for linked series

Adds a new job type that refreshes metadata from external providers for
all series already linked via approved external_metadata_links. Tracks
and displays per-field diffs (series and book level), respects locked
fields, and provides a detailed change report in the job detail page.

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

apps/indexer/src/job.rs

@@ -37,7 +37,7 @@ pub async fn cleanup_stale_jobs(pool: &PgPool) -> Result<()> {
 }
 
 /// Job types processed by the API, not the indexer.
-const API_ONLY_JOB_TYPES: &[&str] = &["metadata_batch"];
+const API_ONLY_JOB_TYPES: &[&str] = &["metadata_batch", "metadata_refresh"];
 
 /// Job types that modify book/thumbnail data and must not run concurrently.
 const EXCLUSIVE_JOB_TYPES: &[&str] = &[