feat(forge): push webhook flags drift at the moment the repo moves (#2691)

Second adapter consumer. POST /api/webhooks/forge validates Gitea's
X-Gitea-Signature (HMAC-SHA256, constant-time; no secret configured =
the endpoint 404s out of existence), extracts changed/removed paths,
and flags matched snippets by writing verification.invalidated_by
{commit_sha, at, path, removed} — the existing attention vocabulary
extended, not a new flag: needs_attention includes it, both filter
dialects (Python + jsonpath SQL) include it in 'attention' and exclude
it from 'ok', and recording ANY fresh verdict clears it by construction
because compose_verification builds a new dict. Unverified snippets are
skipped (already in their own bucket); replayed deliveries at the same
head commit are no-ops; processing failures return 200 with a WARNING +
AppLog canary so the forge never marks deliveries failed and operators
never disable the hook over a transient (#2663's lesson).

Matching goes through repo BINDINGS: recorded location repos are
free-form names ('Scribe') that cannot address a forge, so a snippet
reaches its forge repo through its project's binding — which also fixes
step 5's pull-time resolution for every real record via the same
fallback. O(bindings + snippets-in-project + changed files).

Settings: webhook secret beside the forge config (masked, sentinel-
skipped, Docker-secret env channel, endpoint documented in the UI).
Tests: signature gate, payload parsing, path semantics, both filter
dialects extended in the drift-check guard file, and real-Postgres
end-to-end (flag lands, attention lists it, replay quiet, re-verify
clears, unbound repo untouched).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/routes/settings.py

@@ -20,7 +20,7 @@ settings_bp = Blueprint("settings", __name__, url_prefix="/api/settings")
 # read and skip the mask on write; this generic KV surface has to apply the
 # same treatment, or it silently un-masks what those endpoints masked — the
 # rows live on the admin's own user_id, so the plain GET returned them raw.
-_SECRET_KEYS = frozenset({"smtp_password", "forge_token"})
+_SECRET_KEYS = frozenset({"smtp_password", "forge_token", "forge_webhook_secret"})
 _SECRET_MASK = "********"