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/auth.go

@@ -7,6 +7,7 @@ import (
 	"bookhoard/internal/database"
 	"bookhoard/internal/middleware"
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"os"
@@ -887,7 +888,7 @@ func (h *AuthHandler) DeleteUser(c *echo.Context) error {
 	// Delete user (this will cascade to delete all related data)
 	err = h.db.DeleteUser(c.Request().Context(), targetUserUUID)
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			if c.Request().Header.Get("HX-Request") == "true" {
 				return c.HTML(http.StatusNotFound, `<div class="text-red-500">User not found</div>`)
 			}
@@ -940,7 +941,7 @@ func (h *AuthHandler) UpdateUserMaxDevices(c *echo.Context) error {
 		MaxDevices: pgtype.Int4{Int32: req.MaxDevices, Valid: true},
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "user not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})