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)