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/docs/handler.go

@@ -79,7 +79,7 @@ func (h *DocsHandler) LoadDocument(docPath string) (*templates.Document, error)
 	// Convert markdown to HTML
 	var buf bytes.Buffer
 	context := parser.NewContext()
-	if err := h.markdown.Convert([]byte(content), &buf, parser.WithContext(context)); err != nil {
+	if err := h.markdown.Convert(content, &buf, parser.WithContext(context)); err != nil {
 		return nil, fmt.Errorf("failed to convert markdown: %w", err)
 	}