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>

.gitea/workflows/canary.yml

@@ -45,5 +45,4 @@ jobs:
           CANARY_LOGIN_EMAIL: ${{ secrets.CANARY_LOGIN_EMAIL }}
           CANARY_LOGIN_PASSWORD: ${{ secrets.CANARY_LOGIN_PASSWORD }}
           CANARY_ALERT_TO: ${{ secrets.CANARY_ALERT_TO }}
-          CANARY_SYNTHETIC_KEY: ${{ secrets.CANARY_SYNTHETIC_KEY }}
         run: python3 scripts/canary.py