[bug] Kobo sync times out on large libraries even with shelf-based sync enabled

[rfsbraz]

Kobo sync consistently fails on a large library (~13,000 books) because the sync endpoint takes 30-45+ seconds to respond, exceeding the Kobo e-reader's ~30-second HTTP timeout. This happens even when "Only sync books in selected shelves" is enabled with a
shelf containing only 63 books.

Environment

• CWA version: v4.0.6
• Kobo model: Libra Colour (firmware 4.44)
• Reverse proxy: Traefik
• Library size: ~13,000 books
• Kobo sync shelf: 63 books
• kobo_only_shelves_sync: enabled
• config_kobo_proxy: enabled

Evidence

Traefik access logs show every sync request returns 499 (client disconnect) after ~30s:

"GET /kobo/.../v1/library/sync?Filter=ALL&..." HTTP/1.1" 499 21 ... 29895ms
"GET /kobo/.../v1/library/sync?Filter=ALL&..." HTTP/1.1" 499 21 ... 29712ms
"GET /kobo/.../v1/library/sync?Filter=ALL&..." HTTP/1.1" 499 21 ... 30301ms
"GET /kobo/.../v1/library/sync?Filter=ALL&..." HTTP/1.1" 499 21 ... 30222ms

Requests made from curl (which waits longer) complete successfully with HTTP 200 after 40-80s.

CWA debug logs show each sync step takes 10-13s even for a single changed book:

09:39:31.106 - Kobo library sync request received
09:39:31.205 - Kobo Sync: books last modified (0.1s)
09:39:44.512 - Kobo Sync: changed entries: 1 (13s to scan!)
09:39:55.314 - Kobo Sync: selected to sync: 1 (11s)
09:40:05.344 - Kobo Sync: cover processing (10s)
09:40:15.821 - Kobo Sync: remaining books to sync: 0 (10s)
09:40:15.830 - Kobo Sync: changed states: 1

Total: ~45 seconds to sync 1 book.

The bottleneck is database scanning, not conversion or network

• All books are pre-converted to kepub (ran kepubify --update --inplace --calibre)
• No on-the-fly conversion happening during sync
• Each query step appears to scan the full 13K book database rather than being filtered by the sync shelf

Expected behavior

With shelf-based sync enabled (63 books), the sync queries should be scoped to only those 63 books, completing well within the Kobo's ~30-second timeout.

Steps to reproduce

1. Set up CWA with a large Calibre library (10K+ books)
2. Enable Kobo sync with shelf-based filtering on a small shelf
3. Attempt to sync from a Kobo e-reader
4. Observe "Sync failed" on the Kobo due to timeout

Additional context

• Related to [bug] Kobo not syncing in v4.0.4 #1011 but distinct issue — [bug] Kobo not syncing in v4.0.4 #1011 was about oauth/proxy settings, this is about query performance
• The 8,508 entries in kobo_reading_state table may also contribute to slow queries
• Smaller libraries (< 4K books) reportedly work fine per other users in [bug] Kobo not syncing in v4.0.4 #1011"