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/app/app.go

@@ -2,6 +2,7 @@ package app
 
 import (
 	"context"
+	"errors"
 	"log"
 	"net/http"
 	"os"
@@ -40,7 +41,7 @@ func (a *App) StartServer(addr string) error {
 
 	// Start HTTP server in background
 	go func() {
-		if err := a.server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+		if err := a.server.ListenAndServe(); err != nil && errors.Is(err, http.ErrServerClosed) {
 			log.Fatalf("Server failed to start: %v", err)
 		}
 	}()