fix(storage): emit origin-relative object URLs

Build signed object URLs as path-only (/api/objects/...) instead of
prefixing APP_BASE_URL, so media loads from whatever host or reverse
proxy served the app. Remove the now-unused base URL from the signer and
drop the APP_BASE_URL / Server.BaseURL config.

backend/internal/storage/fs_test.go

@@ -16,7 +16,7 @@ import (
 
 func newTestFS(t *testing.T) *FS {
 	t.Helper()
-	signer, err := NewSigner("http://app.test", "test-secret", time.Hour, 10*time.Minute)
+	signer, err := NewSigner("test-secret", time.Hour, 10*time.Minute)
 	if err != nil {
 		t.Fatalf("signer: %v", err)
 	}