Match Studio HMAC canonicalization

Use the exact newline-separated envelope format produced by the Studio proxy.

app/security/studio_identity.py

@@ -56,7 +56,7 @@ def verify_studio_identity(
     current = time.time() if now is None else now
     if abs(current - timestamp) > max_age_seconds:
         raise ValueError("Studio identity envelope has expired.")
-    canonical = "\\n".join((timestamp_text, subject, email, name)).encode("utf-8")
+    canonical = "\n".join((timestamp_text, subject, email, name)).encode("utf-8")
     expected = hmac.digest(secret.encode("utf-8"), canonical, "sha256").hex()
     if not hmac.compare_digest(expected, signature):
         raise ValueError("Studio identity signature is invalid.")