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/media_scanner.go

@@ -1536,7 +1536,7 @@ func (s *MediaScanner) extractEPUBCover(epubPath string) (string, error) {
 			opfStartAttr += len("full-path=")
 			quote := content[opfStart+opfStartAttr]
 			opfStartQuote := opfStart + opfStartAttr + 1
-			opfEndQuote := bytes.Index(content[opfStartQuote:], []byte{byte(quote)})
+			opfEndQuote := bytes.Index(content[opfStartQuote:], []byte{quote})
 			if opfEndQuote == -1 {
 				continue
 			}