Prevent circular fragment fetching between apps

fetch_fragment() auto-returns "" when called inside a fragment request
(detected via X-Fragment-Request header). This prevents deadlocks when
e.g. blog fetches cart-mini from cart, and cart's context processor
fetches nav-tree from blog.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infrastructure/fragments.py

@@ -59,6 +59,15 @@ def _internal_url(app_name: str) -> str:
 # Public API
 # ------------------------------------------------------------------
 
+def _is_fragment_request() -> bool:
+    """True when the current request is itself a fragment fetch."""
+    try:
+        from quart import request as _req
+        return bool(_req.headers.get(FRAGMENT_HEADER))
+    except Exception:
+        return False
+
+
 async def fetch_fragment(
     app_name: str,
     fragment_type: str,
@@ -72,7 +81,13 @@ async def fetch_fragment(
     Returns the raw HTML string.  When *required* is True (default),
     raises ``FragmentError`` on network errors or non-200 responses.
     When *required* is False, returns ``""`` on failure.
+
+    Automatically returns ``""`` when called inside a fragment request
+    to prevent circular dependencies between apps.
     """
+    if _is_fragment_request():
+        return ""
+
     base = _internal_url(app_name)
     url = f"{base}/internal/fragments/{fragment_type}"
     try: