refactor: use errors.Is()/errors.AsType() for error comparison and rename shadowed variables

Replace direct error equality checks (err == pgx.ErrNoRows, err != http.ErrServerClosed)
with the idiomatic errors.Is() function throughout handlers, services, middleware, and app
startup. This correctly handles wrapped error chains.

Also replace a raw type assertion (*HTTPError) with errors.AsType[*HTTPError]() in the
error handler middleware for consistency.

Additionally, rename shadowed variables for clarity:
- sidecar.go: config -> sidecarConfig, systemConfig (shadowed package-level vars)
- media_scanner.go: uuid -> uuidString (shadowed the uuid package import)

internal/handlers/sidecar.go

@@ -171,7 +171,7 @@ func (h *SidecarHandler) GetSidecarConfig(c *echo.Context) error {
 	}
 
 	// Build sidecar config
-	config := SidecarConfig{
+	sidecarConfig := SidecarConfig{
 		Version: "1.0",
 		Bookhoard: SidecarBookhoardConfig{
 			OPDSCatalog: opdsCatalogURL,
@@ -188,7 +188,7 @@ func (h *SidecarHandler) GetSidecarConfig(c *echo.Context) error {
 		LastUpdated:    time.Now().Format(time.RFC3339),
 	}
 
-	return c.JSON(http.StatusOK, config)
+	return c.JSON(http.StatusOK, sidecarConfig)
 }
 
 // DownloadSidecarConfig generates a .bookhoard.json file for device setup
@@ -352,8 +352,8 @@ func (h *SidecarHandler) GetSystemConfiguration(c *echo.Context) error {
 
 	// Build config map
 	result := make(map[string]string)
-	for _, config := range configs {
-		result[config.Key] = config.Value
+	for _, systemConfig := range configs {
+		result[systemConfig.Key] = systemConfig.Value
 	}
 
 	return c.JSON(http.StatusOK, result)