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>

infra/migrations/0037_add_metadata_refresh_job_type.sql

@@ -0,0 +1,5 @@
+-- Allow metadata_refresh job type in index_jobs
+ALTER TABLE index_jobs
+  DROP CONSTRAINT IF EXISTS index_jobs_type_check,
+  ADD CONSTRAINT index_jobs_type_check
+    CHECK (type IN ('scan', 'rebuild', 'full_rebuild', 'thumbnail_rebuild', 'thumbnail_regenerate', 'cbr_to_cbz', 'metadata_batch', 'metadata_refresh'));