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.

tests/test_backend_conformance.py

@@ -201,11 +201,57 @@ def test_health_passes_on_a_live_backend(backend):
     backend.health()
 
 
+def test_chain_links_and_verify_is_clean(backend):
+    from audit_core.integrity import GENESIS
+
+    first = make_event("chain-1")
+    second = make_event("chain-2")
+    backend.accept(first, digest(first))
+    backend.accept(second, digest(second))
+    report = backend.verify_chain()
+    assert report.intact is True
+    assert report.events >= 2
+    assert report.first_break is None
+    assert report.head != GENESIS
+    replay = backend.accept(first, digest(first))
+    assert replay.duplicate is True
+    assert backend.verify_chain().events == report.events
+
+
 # --- postgres-specific guarantees -------------------------------------------
 
 pg_only = pytest.mark.skipif(not HAVE_PG, reason="needs PostgreSQL")
 
 
+@pg_only
+def test_rewritten_payload_fails_verify_postgres():
+    """Superuser rewrite is the evidence the trigger never gave us."""
+    from audit_core.postgres_backend import PostgresAuditBackend
+
+    schema = f"conf_{uuid.uuid4().hex[:12]}"
+    backend = PostgresAuditBackend(PG_URL, schema=schema)
+    try:
+        event = make_event("break-1")
+        backend.accept(event, digest(event))
+        other = make_event("break-2")
+        backend.accept(other, digest(other))
+        assert backend.verify_chain().intact is True
+        with backend.pool.connection() as conn:
+            conn.execute(f'ALTER TABLE "{schema}".events DISABLE TRIGGER events_append_only')
+            conn.execute(
+                f'UPDATE "{schema}".events SET payload_hash = %s WHERE event_id = %s',
+                ("deadbeef" * 8, "break-2"),
+            )
+            conn.execute(f'ALTER TABLE "{schema}".events ENABLE TRIGGER events_append_only')
+        report = backend.verify_chain()
+        assert report.intact is False
+        assert report.first_break == "break-2"
+    finally:
+        with backend.pool.connection() as conn:
+            conn.execute(f'DROP SCHEMA IF EXISTS "{schema}" CASCADE')
+        backend.close()
+
+
 @pg_only
 def test_replay_reconciles_rather_than_duplicating():
     """Replay must never mint a second custody record for one source event."""