v0.18.0 Slice 3: webhook tightening — mandatory secret + dev-bypass

`GITEA_WEBHOOK_SECRET` is now mandatory at startup. The framework
refuses to load_config() when the env var is empty unless the
operator opts into the dev-bypass with `RFC_APP_INSECURE_WEBHOOKS=1`.
This is the v0.18.0 startup-loud-failure shape — the pre-v0.18.0
"silently accept unsigned POSTs when the secret is empty" path is
the bug the proposal targets.

`webhooks.receive`:
  * Defense in depth: refuses 500 if the secret is empty at request
    time and the dev-bypass is not set (catches the case where
    something mutates env after startup).
  * Logs a loud warning every time a webhook lands under the
    bypass — so a misconfigured production deployment shows up in
    the logs even if the operator missed the warning at boot.
  * Adds an INFO log at the unknown-repo branch (previously
    silently 200-OK'd a hook on a fork or a stale Gitea binding).

`tmp_env` fixture binds a fake secret so the existing 277 tests
boot cleanly; the new test_webhooks_vertical.py exercises both
the production-secret path (valid signature, invalid signature,
missing signature) and the dev-bypass path (config loads with
empty secret when bypass set, refuses without it).

7 new tests; full suite: 284 passed.

backend/tests/test_propose_vertical.py

@@ -438,7 +438,11 @@ def tmp_env(monkeypatch):
         "SECRET_KEY": "test-secret-key-for-cookies",
         "DATABASE_PATH": str(db_path),
         "OWNER_GITEA_LOGIN": "ben",
-        "GITEA_WEBHOOK_SECRET": "",
+        # v0.18.0: `GITEA_WEBHOOK_SECRET` is now mandatory at startup
+        # per the email + webhook hygiene proposal. Tests bind a fake
+        # value so the framework boots; tests that want to exercise
+        # the dev-bypass path monkeypatch `RFC_APP_INSECURE_WEBHOOKS=1`.
+        "GITEA_WEBHOOK_SECRET": "test-webhook-secret-for-signature-verification",
         "ENABLED_MODELS": "claude",
     }
     for k, v in env.items():