[bug] Kobo Sync: books not transferred or not downloadable on device in v4.0.6 despite successful shelf sync

[crocodilestick]

Summary

In CWA v4.0.6, Kobo sync reaches the server correctly and shelves synchronize, but books either never appear on the Kobo or appear but fail to download. Stock linuxserver/calibre-web pointed at the same library syncs successfully, confirming this is CWA-specific.

Symptoms (varies by device/firmware)

• Books never appear on the Kobo after sync (Kobo sync fails with "Sync Failed. Please Try Again" on v4.0.6 despite server sending books correctly #1264, Kobo Sync creates shelves but no books are synchronized (works with Calibre-Web, fails with Calibre-Web Automated v4.0.6) #1413)
• Books appear and show details, but "Download Failed" when trying to read ([bug] Books not downloading to Kobo after sync #1398)
• "Sync Failed. Please Try Again" message after sync completes server-side (Kobo sync fails with "Sync Failed. Please Try Again" on v4.0.6 despite server sending books correctly #1264)

Diagnostic findings (from #1264)

kobo_reading_state, kobo_statistics, and kobo_bookmark tables had duplicate rows for all shelf books, causing SQLAlchemy warnings on every sync:

SAWarning: Multiple rows returned with uselist=False for lazily-loaded attribute 'KoboReadingState.statistics'

kobo_synced_books gets repopulated on container restart before the Kobo connects, causing changed entries: 0 on every subsequent sync attempt.

Environment

• CWA v4.0.6
• Kobo devices: Clara 2E (firmware 4.38), Libra 2, Libra Colour (firmware 4.45)
• Stock Calibre-Web works with identical library and shelf configuration

Related (consolidated from)

• Kobo sync fails with "Sync Failed. Please Try Again" on v4.0.6 despite server sending books correctly #1264 — Kobo sync fails with "Sync Failed. Please Try Again" on v4.0.6
• [bug] Books not downloading to Kobo after sync #1398 — Books not downloading to Kobo after sync
• Kobo Sync creates shelves but no books are synchronized (works with Calibre-Web, fails with Calibre-Web Automated v4.0.6) #1413 — Kobo Sync creates shelves but no books are synchronized