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/system_settings.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"bookhoard/internal/database"
+	"errors"
 	"net/http"
 	"strconv"
 
@@ -47,7 +48,7 @@ func (h *SystemSettingsHandler) UpdateScanSettings(c *echo.Context) error {
 		SettingValue: scanFrequencyValue,
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "system setting not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -58,7 +59,7 @@ func (h *SystemSettingsHandler) UpdateScanSettings(c *echo.Context) error {
 		SettingValue: autoScanValue,
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "system setting not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -74,7 +75,7 @@ func (h *SystemSettingsHandler) UpdateScanSettings(c *echo.Context) error {
 func (h *SystemSettingsHandler) GetScanSettings(c *echo.Context) error {
 	scanFrequencySetting, err := h.db.GetSystemSetting(c.Request().Context(), "scan_poll_interval_seconds")
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusOK, ScanSettingsResponse{
 				ScanPollIntervalSeconds: 60,
 				AutoScanEnabled:         true,
@@ -85,7 +86,7 @@ func (h *SystemSettingsHandler) GetScanSettings(c *echo.Context) error {
 
 	autoScanSetting, err := h.db.GetSystemSetting(c.Request().Context(), "auto_scan_enabled")
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusOK, ScanSettingsResponse{
 				ScanPollIntervalSeconds: 60,
 				AutoScanEnabled:         true,