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/queries/queries.sql

@@ -893,6 +893,10 @@ ORDER BY deleted ASC, deleted_at DESC NULLS LAST
 LIMIT 1;
 
 -- name: CreateMediaBookmarkFull :one
+-- The dedup key can change while the title stays the same (e.g. a client
+-- upgrading a bookmark from percentage-only to a CFI position), so the
+-- title UNIQUE constraint races the dedup-key lookup. Upsert on the title
+-- slot: the conflicting row is by definition the same bookmark.
 INSERT INTO media_bookmarks (
     media_item_id, user_id, page_number, chapter_number,
     cfi_position, title, position, notes,
@@ -901,7 +905,23 @@ INSERT INTO media_bookmarks (
     device_sync_data
 ) VALUES (
     $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15
-) RETURNING *;
+) ON CONFLICT (media_item_id, user_id, title) DO UPDATE SET
+    page_number = EXCLUDED.page_number,
+    chapter_number = EXCLUDED.chapter_number,
+    cfi_position = EXCLUDED.cfi_position,
+    position = EXCLUDED.position,
+    notes = EXCLUDED.notes,
+    percentage_location = EXCLUDED.percentage_location,
+    epubcfi_location = EXCLUDED.epubcfi_location,
+    chapter_reference = EXCLUDED.chapter_reference,
+    dedup_key = EXCLUDED.dedup_key,
+    last_modified_at = EXCLUDED.last_modified_at,
+    last_modified_source = EXCLUDED.last_modified_source,
+    device_sync_data = EXCLUDED.device_sync_data,
+    created_at = created_at,
+    deleted = FALSE,
+    deleted_at = NULL
+RETURNING *;
 
 -- name: UpdateMediaBookmarkForSync :one
 UPDATE media_bookmarks SET