Switch federation link-card fragment to sexp rendering

All four services (blog, market, events, federation) now use the shared
~link-card s-expression component instead of per-service Jinja templates.

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

federation/bp/fragments/routes.py

@@ -9,6 +9,7 @@ from __future__ import annotations
 from quart import Blueprint, Response, request
 
 from shared.infrastructure.fragments import FRAGMENT_HEADER
+from shared.sexp.jinja_bridge import sexp
 
 
 def register():
@@ -31,8 +32,20 @@ def register():
 
     # --- link-card fragment: actor profile preview card --------------------------
 
+    def _render_federation_link_card(actor, link: str) -> str:
+        return sexp(
+            '(~link-card :link link :title title :image image'
+            ' :icon "fas fa-user" :subtitle username'
+            ' :detail summary :data-app "federation")',
+            link=link,
+            title=actor.display_name or actor.preferred_username,
+            image=None,
+            username=f"@{actor.preferred_username}" if actor.preferred_username else None,
+            summary=actor.summary,
+        )
+
     async def _link_card_handler():
-        from quart import g, render_template
+        from quart import g
         from shared.services.registry import services
         from shared.infrastructure.urls import federation_url
 
@@ -48,13 +61,8 @@ def register():
                 parts.append(f"<!-- fragment:{u} -->")
                 actor = await services.federation.get_actor_by_username(g.s, u)
                 if actor:
-                    parts.append(await render_template(
+                    parts.append(_render_federation_link_card(
-                        "fragments/link_card.html",
+                        actor, federation_url(f"/users/{actor.preferred_username}"),
-                        display_name=actor.display_name,
-                        username=actor.preferred_username,
-                        avatar_url=None,
-                        summary=actor.summary,
-                        link=federation_url(f"/users/{actor.preferred_username}"),
                     ))
             return "\n".join(parts)
 
@@ -65,13 +73,8 @@ def register():
         actor = await services.federation.get_actor_by_username(g.s, lookup)
         if not actor:
             return ""
-        return await render_template(
+        return _render_federation_link_card(
-            "fragments/link_card.html",
+            actor, federation_url(f"/users/{actor.preferred_username}"),
-            display_name=actor.display_name,
-            username=actor.preferred_username,
-            avatar_url=None,
-            summary=actor.summary,
-            link=federation_url(f"/users/{actor.preferred_username}"),
         )
 
     _handlers["link-card"] = _link_card_handler