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>

scripts/canary.py

@@ -73,10 +73,9 @@ class Check:
 def _probe(c: Check) -> Result:
     data = json.dumps(c.body).encode() if c.body else None
     headers = {"User-Agent": "windy-git-canary/1.0", **c.headers}
-    # Mark our own probes so the ledger can tell a canary forgery from an attack.
-    # A shared secret, not a flag: a bare header would let an attacker hide.
-    if os.environ.get("CANARY_SYNTHETIC_KEY"):
-        headers["X-Windy-Synthetic"] = os.environ["CANARY_SYNTHETIC_KEY"]
+    # Our own probes are synthetic traffic (ecosystem convention, Telemetry
+    # UPDATE 4): every service they touch labels the resulting rows.
+    headers["X-Windy-Synthetic"] = "1"
     if data:
         headers["Content-Type"] = "application/json"
     req = urllib.request.Request(c.url, data=data, method=c.method, headers=headers)