fix(backend): swap db name in URL-style test DSN

The integration suite creates a per-run database and rewrites the admin
DSN to point at it via replaceDSNDatabase. The URL-style branch was a
stub that returned the DSN unchanged, so with a URL DSN every test ran
against the shared `postgres` database instead of its own — no isolation.
The first test to create a given user/object won; the rest collided on
unique constraints, surfacing as cascades of 500s (and ImportFromFolder
seeing accumulated rows).

CI passes TANABATA_TEST_ADMIN_DSN in URL form, so this only broke in the
deploy pipeline, not local key=value runs. Parse the URL and swap its
path to the per-run database.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/integration/server_test.go

@@ -1614,7 +1614,13 @@ func replaceDSNDatabase(dsn, newDB string) string {
 		}
 		return dsn + " dbname=" + newDB
 	}
-	// URL style: not used in our defaults, but handled for completeness.
+	// URL style (e.g. postgres://user:pass@host:port/dbname?opts): the database
+	// is the URL path. CI passes this form via TANABATA_TEST_ADMIN_DSN, so it
+	// must swap the path to point the suite at its per-run database.
+	if u, err := url.Parse(dsn); err == nil {
+		u.Path = "/" + newDB
+		return u.String()
+	}
 	return dsn
 }