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/services/conversion_service.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"bookhoard/internal/database"
+
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
@@ -81,7 +82,10 @@ func (s *ConversionService) ConvertEPUBToKEPUB(ctx context.Context, mediaItemID
 	}
 
 	var fileSize pgtype.Int8
-	fileSize.Scan(int64(fileinfo.Size()))
+	err = fileSize.Scan(fileinfo.Size())
+	if err != nil {
+		return nil, err
+	}
 
 	_, err = s.db.CreateMediaItemFormat(ctx, database.CreateMediaItemFormatParams{
 		MediaItemID:           mediaItemID,