* fix(collections): sync matches across all bound libraries
Collection sync resolved item membership against only the legacy
library_collections.library_id, so a collection bound to multiple
libraries (N:N library_collection_libraries) had its items collapsed to a
single library on every sync.
Switch the MDBList/TMDB/Trakt sync matchers to the existing
LibraryItemRepository.GetItemsInFolders over collection.LibraryIDs.
GetByID already populates LibraryIDs from the N:N table (falling back to
[LibraryID]), so single-library collections are unaffected. No schema
change; listing and the catalog open path were already N:N-aware.
Unblocks merging duplicated per-library collections into single
multi-library collections.
* fix(collections): dedupe matched items by media_item_id before ReplaceItems
A collection sync resolves source entries (external IDs) to library
content_ids and appends one item per matched entry without deduping by
media_item_id. When two different source entries resolve to the same
library item, the same content_id is appended twice; ReplaceItems then
hits the library_collection_items composite PK (collection_id,
media_item_id) with a duplicate-key violation, failing the whole sync
with an HTTP 500 and zero items.
The multi-library union added in the previous commit (GetItemsInFolders
over collection.LibraryIDs) widens the matched set and so raises the
collision probability, making this latent bug more likely to fire.
Dedupe at the single choke point in ReplaceItems (first occurrence wins,
preserving rank order) so all five sync paths are covered at once, and
renumber position over the surviving rows so it stays dense.
Also log the underlying error in HandleSyncAdminCollection, which
previously mapped every non-sentinel sync error to a generic 500 with
no log line, making this failure invisible on the server.
