Add frequent thin scrape for actively touring artists

[switz]

Summary

• Adds a recurring Hangfire job (RefreshActiveArtists) that runs every ~2 hours (0, 2, 4, 12, 14, 16, 18, 20, 22 UTC — avoiding the 6-11 AM daily import window)
• Finds artists with a show in the past 7 days and runs a lightweight "thin scrape" limited to the current year
• Introduces ImportOptions to thread thin-scrape behavior through the importer pipeline without changing abstract method signatures

Per-importer optimizations

• Archive.org: Appends year:<current> to the Lucene search query so only current-year recordings are fetched; skips source-deletion logic (since results are a filtered subset)
• Phish.in: Paginates from the last page backward (newest shows first), skipping pre-cutoff shows and stopping once an entire page has no in-range shows. Eras/tours/songs/venues still sync to avoid KeyNotFoundException on new entities.
• PhishNet: Filters source list to current-year display_date only, skipping per-show rating scrapes for historical shows

Test plan

• Build succeeds, all 52 existing tests pass
• Trigger RefreshActiveArtists via Hangfire dashboard with allowedInDev=true — verify it identifies artists with recent shows
• Verify thin scrape for Phish processes only current-year shows (check logs for page count and early stop message)
• Verify archive.org thin scrape URL includes year filter
• Verify no historical sources are deleted during thin scrape
• Verify artistsCurrentlySyncing guard prevents overlap between thin scrape and full daily import