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/config.py

@@ -71,9 +71,6 @@ class Settings(BaseSettings):
     # root-only /etc/windygit/telemetry.env on Veron, never in the repo.
     windygit_telemetry_token: str = ""
     telemetry_ingest_url: str = "https://admin.windyword.ai/v1/events"
-    # Shared with our canary (Gitea repo secret CANARY_SYNTHETIC_KEY). A request
-    # carrying it in X-Windy-Synthetic is our own tooling -> synthetic:true.
-    windygit_synthetic_key: str = ""
 
     # Internal callers (the Cloud portal calling /internal/*). A first-class
     # caller class, not a bypass: unset means service calls are REFUSED.