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/progress.go

@@ -5,6 +5,7 @@ import (
 	wsync "bookhoard/internal/sync"
 	"bookhoard/internal/utils"
 	"context"
+	"errors"
 	"net/http"
 	"strconv"
 	"time"
@@ -43,7 +44,7 @@ func (h *Handler) GetUniversalProgress(c *echo.Context) error {
 		UserID:      pgtype.UUID{Bytes: user.ID.Bytes, Valid: true},
 	})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusOK, map[string]interface{}{
 				"media_item_id": mediaItemID,
 				"progress":      nil,