audit-3: append-only audit log + strict legacy-prefix rule + anchor cross-check

- verify_audit_chain(): rows with entry_hash IS NULL are only tolerated
  BEFORE the first hashed row (documented legacy prefix). A NULL hash
  AFTER the chain started is tampering and fails verification at that
  row id — previously any NULL row anywhere was silently skipped, so an
  attacker with UPDATE could erase intermediate evidence undetected.
- migration 0004: audit_log becomes append-only. SQLite: BEFORE UPDATE/
  DELETE triggers RAISE(ABORT). Postgres: BEFORE triggers RAISE EXCEPTION.
  Closes the hole for the table owner and direct SQL connections.
- monitor.verify_chain_against_anchors(): compares the chain tail against
  anchors published to Nostr. Explicit statuses: no_anchors (never
  anchored / notes missing on relays), local_only (no fetcher), ok
  (relay note attests the tail or the last local anchor), mismatch.
- tests: NULL-before-genesis ok, NULL-in-middle fails at that id, NULL
  late fails, UPDATE rejected, DELETE rejected, INSERT still allowed,
  no-anchors explicit, local-only explicit, mismatch detected, match ok.

Gate: ruff 0 · 81 unit + 1 PG integration · alembic 0001→0004 on PG ·
gitleaks clean · make check exit 0.

alembic/versions/0004_audit_append_only.py

@@ -0,0 +1,89 @@
+"""audit_log append-only: reject UPDATE and DELETE (round-2 hardening)
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-10-28
+
+An audit trail that can be edited is not an audit trail. This migration
+installs, on every engine:
+
+- SQLite: BEFORE UPDATE/DELETE triggers that RAISE(ABORT).
+- Postgres: BEFORE UPDATE/DELETE triggers that RAISE EXCEPTION.
+
+The writer roles never had UPDATE/DELETE grants in roles.sql; this closes
+the hole for the table owner and for direct SQL connections.
+"""
+
+from __future__ import annotations
+
+from alembic import op
+
+revision = "0004"
+down_revision = "0003"
+branch_labels = None
+depends_on = None
+
+SQLITE_TRIGGERS = [
+    """
+    CREATE TRIGGER IF NOT EXISTS audit_log_no_update
+    BEFORE UPDATE ON audit_log
+    BEGIN
+      SELECT RAISE(ABORT, 'cayce-ops: audit_log is append-only (UPDATE rejected)');
+    END;
+    """,
+    """
+    CREATE TRIGGER IF NOT EXISTS audit_log_no_delete
+    BEFORE DELETE ON audit_log
+    BEGIN
+      SELECT RAISE(ABORT, 'cayce-ops: audit_log is append-only (DELETE rejected)');
+    END;
+    """,
+]
+
+PG_TRIGGERS = """
+CREATE OR REPLACE FUNCTION cayce_ops_audit_append_only() RETURNS trigger AS $$
+BEGIN
+  RAISE EXCEPTION 'cayce-ops: audit_log is append-only (% rejected)', TG_OP;
+END;
+$$ LANGUAGE plpgsql;
+"""
+
+
+def upgrade() -> None:
+    bind = op.get_bind()
+    if bind.dialect.name == "postgresql":
+        op.execute(PG_TRIGGERS)
+        op.execute(
+            "DROP TRIGGER IF EXISTS audit_log_no_update ON audit_log;"
+        )
+        op.execute(
+            "CREATE TRIGGER audit_log_no_update BEFORE UPDATE ON audit_log "
+            "FOR EACH ROW EXECUTE FUNCTION cayce_ops_audit_append_only();"
+        )
+        op.execute(
+            "DROP TRIGGER IF EXISTS audit_log_no_delete ON audit_log;"
+        )
+        op.execute(
+            "CREATE TRIGGER audit_log_no_delete BEFORE DELETE ON audit_log "
+            "FOR EACH ROW EXECUTE FUNCTION cayce_ops_audit_append_only();"
+        )
+    else:
+        for ddl in SQLITE_TRIGGERS:
+            bind.execute(sa_text(ddl))
+
+
+def sa_text(sql: str):
+    import sqlalchemy as sa
+
+    return sa.text(sql)
+
+
+def downgrade() -> None:
+    bind = op.get_bind()
+    if bind.dialect.name == "postgresql":
+        op.execute("DROP TRIGGER IF EXISTS audit_log_no_update ON audit_log;")
+        op.execute("DROP TRIGGER IF EXISTS audit_log_no_delete ON audit_log;")
+        op.execute("DROP FUNCTION IF EXISTS cayce_ops_audit_append_only();")
+    else:
+        bind.execute(sa_text("DROP TRIGGER IF EXISTS audit_log_no_update;"))
+        bind.execute(sa_text("DROP TRIGGER IF EXISTS audit_log_no_delete;"))