feat(coverage): pattern-library coverage measurement (#2692, milestone 288 step 7)

Server-side shape enumeration per bound repo — one archive download via the
forge adapter, definitions extracted with a Python mirror of the write-path
hook's awk rules (shared test vectors pin the two together) — compared
against recorded snippet locations by path+symbol. Summary is cached in the
settings KV with a freshness stamp; recomputed on webhook push (spawned off
the delivery path) or explicit refresh, never in a request path.

Surfaces: GET/POST /api/projects/<id>/coverage[/refresh], a project-page
card (estimate-labeled, largest-gaps chips), and a one-line evidence-carrying
entry in enter_project read from cache only.

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

src/scribe/routes/webhooks.py

@@ -29,7 +29,9 @@ import traceback
 from quart import Blueprint, jsonify, request
 
 from scribe.config import Config
-from scribe.services.repo_bindings import normalize_repo_key
+from scribe.services.background import spawn
+from scribe.services.coverage import refresh_coverage
+from scribe.services.repo_bindings import bindings_for_key, normalize_repo_key
 from scribe.services.settings import get_admin_setting
 from scribe.services.snippets import invalidate_for_push
 
@@ -88,6 +90,16 @@ async def forge_push():
             logger.info(
                 "forge push %s flagged %d snippet(s) for recheck", head[:12], flagged
             )
+        # A push is exactly when the coverage number goes stale — recompute it
+        # off the delivery path (#2692). Fire-and-forget: the forge's delivery
+        # loop must not wait on an archive download, and a failure is a
+        # WARNING from spawn(), never a failed delivery. This also SEEDS the
+        # cache on a webhook-configured instance — no manual first refresh.
+        for binding in await bindings_for_key(repo_key):
+            spawn(
+                refresh_coverage(binding.user_id, binding.project_id),
+                site="webhooks.coverage_refresh",
+            )
         return jsonify({"ok": True, "flagged": flagged})
     except Exception:
         logger.warning("forge webhook processing failed", exc_info=True)