Fix error page loop + account startup timeout

- Error handlers for FragmentError and generic Exception now return
  self-contained HTML (no render_template) to avoid the infinite loop
  where context processor → fetch_fragments → error → render_template
  → context processor → fetch_fragments → error ...
- Account Ghost membership sync moved to background task so it doesn't
  block Hypercorn's startup timeout (was causing crash-loop).

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

account/app.py

@@ -82,18 +82,22 @@ def create_app() -> "Quart":
     from bp.data.routes import register as register_data
     app.register_blueprint(register_data())
 
-    # --- Ghost membership sync at startup ---
+    # --- Ghost membership sync at startup (background) ---
+    # Runs as a background task to avoid blocking Hypercorn's startup timeout.
     @app.before_serving
-    async def _sync_ghost_membership():
-        from services.ghost_membership import sync_all_membership_from_ghost
-        from shared.db.session import get_session
-        try:
-            async with get_session() as s:
-                await sync_all_membership_from_ghost(s)
-                await s.commit()
-            print("[account] Ghost membership sync complete")
-        except Exception as e:
-            print(f"[account] Ghost membership sync failed (non-fatal): {e}")
+    async def _schedule_ghost_membership_sync():
+        import asyncio
+        async def _sync():
+            from services.ghost_membership import sync_all_membership_from_ghost
+            from shared.db.session import get_session
+            try:
+                async with get_session() as s:
+                    await sync_all_membership_from_ghost(s)
+                    await s.commit()
+                print("[account] Ghost membership sync complete")
+            except Exception as e:
+                print(f"[account] Ghost membership sync failed (non-fatal): {e}")
+        asyncio.get_event_loop().create_task(_sync())
 
     return app