fix(bookmarks): upsert on title conflict so position upgrades don't 500

Bookmark dedup is keyed on hash(title + position bucket), but the table
also enforces UNIQUE(media_item_id, user_id, title). When a client re-
saves the same bookmark title with a changed position form - e.g. the
Android app upgrading a percentage-only row to an EPUB CFI, or a web and
app bookmark landing on the same 'Bookmark at 44%' title - the dedup-key
lookup misses and the INSERT violates the title constraint, returning
HTTP 500 and failing the sync.

A title collision on the same (user, item) is by definition the same
bookmark slot, so take the LWW semantics all the way: ON CONFLICT DO
UPDATE replaces position/cfi_position/page/chapter/percentage, refreshes
dedup_key and timestamps, merges device_sync_data, and - matching
UpdateMediaBookmarkForSync - clears deleted/deleted_at so a re-create
resurrects a tombstoned title slot instead of leaving an invisible row
holding it.

Device sync flows are unaffected: KOReader/Kobo pushes that carry their
own dedup-key echoes never reach the INSERT, and same-key saves still go
through applyBookmarkLWW with its tombstone freshness checks.

internal/database/db.go

@@ -1,6 +1,6 @@
 // Code generated by sqlc. DO NOT EDIT.
 // versions:
-//   sqlc v1.30.0
+//   sqlc v1.31.1
 
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