chore(api): add /v1/me/ping sanity probe

confirms whether new GET routes under /me/* deploy correctly to
vercel — diagnostic in the middle of the /me/topics 404 chase.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/api/src/modules/mesh/v1-router.ts

@@ -496,6 +496,13 @@ export const v1Router = new Hono<Env>()
     });
   })
 
+  // GET /v1/me/ping — sanity probe for cross-mesh route health.
+  .get("/me/ping", async (c) => {
+    const key = c.var.apiKey;
+    requireCapability(key, "read");
+    return c.json({ ok: true, hasIssuer: !!key.issuedByMemberId });
+  })
+
   // GET /v1/me/topics — cross-mesh topic list for the caller's user.
   //
   // For each topic across every mesh the user belongs to, returns