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/middleware/error_handler.go

@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -83,7 +84,7 @@ func WrapHandler(fn func(*echo.Context) error) echo.HandlerFunc {
 	return func(c *echo.Context) error {
 		err := fn(c)
 		if err != nil {
-			if httpErr, ok := err.(*HTTPError); ok {
+			if httpErr, ok := errors.AsType[*HTTPError](err); ok {
 				return RespondWithHTTPError(c, httpErr)
 			}
 			return RespondWithError(c, http.StatusInternalServerError, "Internal server error", err)