Open Library Solr Bulk Indexing BWB & Lenny OPDS (price)

[P2/P1] Feature: Index and harvest a curated list of OPDS feeds on a schedule, update Open Library and Solr

### Problem
Open Library needs a mechanism to index/harvest a manually curated list of OPDS URL feeds. Currently, there is no system to repeatedly check and update these feeds within Open Library and Solr.

### Solution Proposal
- Allow Open Library administrators to maintain a curated list of OPDS feed URLs
- Implement a daily cron job that:
- Checks each OPDS feed for updates (define: hash, modified date, or other)
- Detects changes since last check
- Updates Open Library and pushes changes to Solr

### Requirements
- Method to determine when an OPDS feed has changed (e.g., ETag, Last-Modified, checksum, etc.)
- Efficient strategy for update detection (should not re-harvest if unchanged)
- Update or insert new entries into Open Library and Solr index

### Open Questions
- What criteria should be used to determine if an OPDS feed has changed?
- Should this only support Atom OPDS feeds, or also RSS?
- Where to store the status/history of the last harvested state per feed?

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**Priority:** P2/P1 (High)