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/services/search.go

@@ -3,6 +3,7 @@ package services
 import (
 	"bookhoard/internal/database"
 	"context"
+	"errors"
 	"strings"
 
 	"github.com/jackc/pgx/v5"
@@ -214,7 +215,7 @@ func (s *SearchService) SearchFieldValues(ctx context.Context, params FieldSearc
 // Shared between JSON endpoint and HTML rendering
 func (s *SearchService) ExecuteSearch(ctx context.Context, params SearchParams) ([]database.SearchMediaItemsUnifiedRow, int, error) {
 	results, err := s.SearchMediaItemsUnified(ctx, params)
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return nil, 0, err
 	}