refactor: remove unnecessary type conversions and handle ignored errors across codebase

Remove redundant type conversions that Go 1.26 makes unnecessary or that
were already no-ops:

- uuid.UUID(x.Bytes) → x.Bytes (uuid.UUID is [16]byte, same as pgtype UUID Bytes)
- pgtype.UUID{Bytes: [16]byte(u), Valid: true} → pgtype.UUID{Bytes: u, Valid: true}
- (*time.Time)(&x.Time) → &x.Time
- json.RawMessage(x) → x where x is already []byte
- []byte(stringVal) → stringVal where []byte is expected
- int()/int64()/byte() casts on values already of the target type
- Decompressor(fn) → fn (type is identical)

Handle previously ignored error returns:

- collections.go: check json.Unmarshal error in GetCollection
- conversion_service.go: check fileSize.Scan() error
- app_test.go: check app.Shutdown() error in benchmark

internal/handlers/kobo.go

@@ -33,7 +33,7 @@ func (h *KoboHandler) mapContentIdToBookhoardUUID(ctx *echo.Context, contentId s
 	catalog, err := h.db.GetDeviceCatalogByKoboContentId(ctx.Request().Context(), contentId)
 	if err == nil && catalog.ID.Valid {
 		// Found! Use canonical Bookhoard UUID
-		return uuid.UUID(catalog.BookhoardUuid.Bytes), nil, "catalog_match"
+		return catalog.BookhoardUuid.Bytes, nil, "catalog_match"
 	}
 
 	// Step 2: ContentId not found - check if it looks like a SHA-256 hash
@@ -52,7 +52,7 @@ func (h *KoboHandler) mapContentIdToBookhoardUUID(ctx *echo.Context, contentId s
 				DeliveryDate:   pgtype.Timestamptz{Time: time.Now(), Valid: true},
 				DeliveryMethod: pgtype.Text{String: "sync", Valid: true},
 			})
-			return uuid.UUID(mediaItem.ID.Bytes), nil, "sha256_match"
+			return mediaItem.ID.Bytes, nil, "sha256_match"
 		}
 	}