Implement AUDIT-WP-0007 hash-chain integrity.

Accept now extends a single-schema chain. Verify walks it; a rewritten
payload_hash is a break. Tamper evidence is that detector plus an
external chain-head attestation, not WORM.

audit_core/ingestion.py

@@ -159,7 +159,12 @@ class IngestionApplication:
 
         if method == "GET" and (
             path.startswith("/v1/events")
-            or path in ("/v1/dead-letters", "/v1/secret-findings", "/v1/stats")
+            or path in (
+                "/v1/dead-letters",
+                "/v1/secret-findings",
+                "/v1/stats",
+                "/v1/integrity",
+            )
         ):
             return self._read(start_response, environ, path, identity)
 
@@ -241,6 +246,14 @@ class IngestionApplication:
                     start_response, HTTPStatus.OK,
                     {"secret_findings": self.backend.secret_findings(_limit(query))},
                 )
+            if path == "/v1/integrity":
+                verify = getattr(self.backend, "verify_chain", None)
+                if not callable(verify):
+                    return self._json(
+                        start_response, HTTPStatus.NOT_FOUND,
+                        {"error": "integrity_not_supported"},
+                    )
+                return self._json(start_response, HTTPStatus.OK, verify().as_dict())
             if path == "/v1/events":
                 correlation = (query.get("correlation_id") or [""])[0]
                 if not correlation: