Sync playback progress to Hardcover

[jules2689]

Feature request

• Add playback progress syncing for Hardcover, not just status updates.
• BookPlayer currently appears to mark a matched Hardcover book as “Currently Reading” after the reading threshold, and “Read” when finished.
• It would be useful if BookPlayer also updated the active Hardcover read record with the current audiobook position during playback.

I know #1362 (comment) says that it's not possible yet, but I noticed other apps like Grimmory do this now. I've linked their code below.

Narrative

GIVEN:

• I have connected Hardcover in BookPlayer.
• A local audiobook is matched to a Hardcover book.
• I am listening to the audiobook in BookPlayer.

WHEN:

• Playback progresses past the configured Hardcover reading threshold.
• Playback continues over time.
• The audiobook is eventually finished.

THEN:

• The book should be marked as “Currently Reading” in Hardcover, as it is today.
• BookPlayer should periodically sync the current listening position to Hardcover during playback.
• For audiobooks, this could use Hardcover’s user_book_read progress field, ideally progress_seconds.
• When the book is finished, BookPlayer should send the final progress and mark the Hardcover read as finished / read.

Design

Possible implementation approach:

• Keep the existing HardcoverService progress subscription.
• After the reading threshold is crossed, ensure the book exists in the user’s Hardcover library as reading.
• Find or create an open user_book_read record for the Hardcover userBookID.
• During playback, throttle progress updates so Hardcover is not called too often, for example every 60 seconds or only when progress changes meaningfully.
• Send the local playback position as progress_seconds.
• On finish, update the read record one last time with final progress_seconds and finished_at, then mark the book as read.

Other projects appear to do similar Hardcover progress syncing:

• Grimmory syncs progress to Hardcover and updates or inserts user_book_read entries:
  • https://github.com/grimmory-tools/grimmory/blob/be574d5b1638f23873c61fe3d250f96105ed27a8/backend/src/main/java/org/booklore/service/hardcover/HardcoverSyncService.java#L54-L193
• Audiobookshelf Hardcover Sync uses DatesReadInput with progress_seconds and updates user_book_read:
  • https://github.com/drallgood/audiobookshelf-hardcover-sync/blob/f7e21369bf1bee3de1490cb47eeea8140ef04eb5/internal/api/hardcover/client.go#L1691-L1699
  • https://github.com/drallgood/audiobookshelf-hardcover-sync/blob/f7e21369bf1bee3de1490cb47eeea8140ef04eb5/internal/api/hardcover/client.go#L2086-L2115