feat(sync): add shared BookResolver with format-aware SHA-256 matching

The platform had three duplicated, divergent book resolvers (koreader,
kobo, BookMatchingService) and none of them consulted
media_item_formats.file_sha256 - per-format hashes for converted files
(KEPUB, PDF) are computed and stored at import/conversion time but were
never used for lookup. GetMediaItemFormatBySHA256 existed with zero
callers. Any client holding a converted file could never match by
hash.

Add internal/services/book_resolver.go: a single shared resolution
path from client-supplied identifier to media_item.
ResolveBySHA256 checks media_items.file_sha256 first (indexed
GetMediaItemBySHA256), then falls back to media_item_formats.
file_sha256 (indexed GetMediaItemFormatBySHA256, first caller) so a
converted format matches with equal confidence. The import-time
SHA-256 is the canonical identifier shared by every interface.

Wire two of the existing resolvers through it:

- BookMatchingService.matchBySHA256: replaces the in-memory
  ListMediaItems scan of up to 1000 rows with the resolver's indexed
  lookups, and gains format awareness for the link/auto-link UI.
  MatchMethod now reports sha256_sha256 or sha256_sha256_format
- KoboHandler.mapContentIdToBookhoardUUID: the SHA-256 heuristic
  branch (ContentId that looks like a 64-char hash) now resolves
  format-aware too. Kobo's entitlement_id wire identity is untouched;
  only the opportunistic hash branch changed

internal/handlers/kobo.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"bookhoard/internal/database"
+	"bookhoard/internal/services"
 	wsync "bookhoard/internal/sync"
 	"encoding/json"
 	"fmt"
@@ -22,10 +23,11 @@ type KoboHandler struct {
 	progressSvc    *wsync.ProgressService
 	annotationSvc  *wsync.AnnotationService
 	libraryService LibraryPathResolver
+	bookResolver   *services.BookResolver
 }
 
 func NewKoboHandler(db *database.Queries, connManager *wsync.ConnectionManager) *KoboHandler {
-	return &KoboHandler{db: db, connManager: connManager}
+	return &KoboHandler{db: db, connManager: connManager, bookResolver: services.NewBookResolver(db)}
 }
 
 func (h *KoboHandler) SetProgressService(svc *wsync.ProgressService) {
@@ -52,8 +54,9 @@ func (h *KoboHandler) mapContentIdToBookhoardUUID(ctx *echo.Context, contentId s
 
 	// Step 2: ContentId not found - check if it looks like a SHA-256 hash
 	if len(contentId) == 64 && looksLikeSHA256(contentId) {
-		// Try to find media item by SHA-256
-		mediaItem, err := h.db.GetMediaItemBySHA256(ctx.Request().Context(), pgtype.Text{String: contentId, Valid: true})
+		// Try to find media item by SHA-256 (format-aware: also checks
+		// media_item_formats, so a converted/alternate format hash matches).
+		mediaItem, _, err := h.bookResolver.ResolveBySHA256(ctx.Request().Context(), contentId)
 		if err == nil {
 			// Found by SHA-256! Create device catalog entry for future lookups
 			_, _ = h.db.CreateDeviceCatalog(ctx.Request().Context(), database.CreateDeviceCatalogParams{