telemetry: adopt the end-to-end synthetic convention (UPDATE 4)

Replaces the keyed marker from 1c3b5b0 with the ecosystem convention:
any X-Windy-Synthetic value marks the request synthetic; the flag lives in
a per-request contextvar, labels this request's rows, and is FORWARDED on
downstream calls (Eternitas trust lookup, Gitea API). The canary sends
"1". Rows are still recorded; the label separates, never suppresses.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/app/services/gitea_client.py

@@ -20,6 +20,7 @@ import httpx
 
 from api.app.config import Settings
 from api.app.errors import RepairPointer, provider_unconfigured
+from api.app.telemetry import synthetic_headers
 
 _TIMEOUT = httpx.Timeout(20.0, connect=5.0)
 
@@ -36,6 +37,7 @@ class GiteaClient:
         return {
             "Authorization": f"token {self._s.gitea_admin_token}",
             "Content-Type": "application/json",
+            **synthetic_headers(),  # end-to-end synthetic convention (Telemetry UPDATE 4)
         }
 
     async def _request(self, method: str, path: str, **kw: Any) -> httpx.Response: