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.

cayceops/components/monitor/monitor.py

@@ -251,3 +251,120 @@ def build(cfg: Config | None = None) -> int:
         return 0
     finally:
         engine().dispose()
+
+
+def _latest_chain_hash(sess) -> str | None:
+    """Last non-NULL entry_hash in the audit chain (None if chain empty)."""
+    from cayceops.models import AuditEntry
+
+    last = (
+        sess.query(AuditEntry)
+        .filter(AuditEntry.entry_hash.isnot(None))
+        .order_by(AuditEntry.id.desc())
+        .first()
+    )
+    return last.entry_hash if last else None
+
+
+def _published_anchors(sess) -> list[dict]:
+    """Anchor audit entries recorded locally, oldest first."""
+    from cayceops.models import AuditEntry
+
+    rows = (
+        sess.query(AuditEntry)
+        .filter(AuditEntry.action == "anchor")
+        .order_by(AuditEntry.id.asc())
+        .all()
+    )
+    return [
+        {
+            "event_id": (r.detail or {}).get("event_id"),
+            "last_hash": (r.detail or {}).get("last_hash"),
+            "last_entry": (r.detail or {}).get("last_entry"),
+            "ts": r.ts.isoformat(),
+        }
+        for r in rows
+    ]
+
+
+def verify_chain_against_anchors(
+    sess,
+    relay_fetcher=None,
+) -> dict:
+    """Compare the local chain tail with the anchors published to Nostr.
+
+    relay_fetcher(deployment) -> list of note contents (strings) found on
+    the relays for this deployment's anchor tag. When None, only local
+    coherence is checked and the result says so.
+
+    Returns a report dict:
+      status: "no_anchors" | "local_only" | "ok" | "mismatch"
+      detail: human-readable explanation
+      local_last_hash, local_anchors, relay_notes (when fetched)
+    """
+    local_hash = _latest_chain_hash(sess)
+    anchors = _published_anchors(sess)
+
+    report: dict = {
+        "local_last_hash": local_hash,
+        "local_anchors": anchors,
+        "relay_notes": None,
+    }
+
+    if not anchors:
+        report["status"] = "no_anchors"
+        report["detail"] = (
+            "the chain has never been anchored to Nostr — no external "
+            "attestation exists yet (anchor once with `monitor anchor`)"
+        )
+        return report
+
+    last_anchor = anchors[-1]
+    if relay_fetcher is None:
+        report["status"] = "local_only"
+        report["detail"] = (
+            "compared only against the LOCAL record of past anchors "
+            "(no relay fetch configured); last anchored hash "
+            f"{last_anchor['last_hash']}"
+        )
+        return report
+
+    from cayceops.config import load as _load
+
+    deployment = _load().deployment_id
+    notes = relay_fetcher(deployment) or []
+    report["relay_notes"] = notes
+
+    if not notes:
+        report["status"] = "no_anchors"
+        report["detail"] = (
+            "local anchor records exist but no anchor notes were found on "
+            "the relays — the external attestation is missing or was deleted"
+        )
+        return report
+
+    # the freshest relay note must attest the current chain tail
+    expected_fragment = f"last_hash={local_hash}"
+    latest_note = notes[-1]
+    if expected_fragment in latest_note:
+        report["status"] = "ok"
+        report["detail"] = "latest relay anchor matches the current chain tail"
+        return report
+
+    # maybe the chain advanced after the last anchor — that's fine ONLY if
+    # the note matches the hash recorded in the last local anchor entry
+    if last_anchor["last_hash"] and last_anchor["last_hash"] in latest_note:
+        report["status"] = "ok"
+        report["detail"] = (
+            "relay anchor matches the last local anchor; the chain has "
+            "advanced since (anchor again to attest the new tail)"
+        )
+        return report
+
+    report["status"] = "mismatch"
+    report["detail"] = (
+        f"relay anchor does not attest chain tail {local_hash} nor the last "
+        f"local anchor {last_anchor['last_hash']} — the published anchor "
+        "does not correspond to this database"
+    )
+    return report

cayceops/models.py

@@ -209,14 +209,21 @@ def _build_entry(
 def verify_audit_chain(sess) -> tuple[bool, int | None]:
     """Re-walk the chain. Returns (ok, first_broken_entry_id or None).
 
-    Legacy rows (entry_hash IS NULL, written before migration 0002) are
-    skipped and documented: the chain begins GENESIS at the first hashed row,
-    so a migrated database verifies clean without any tampering.
+    Rows with entry_hash IS NULL are ONLY tolerated BEFORE the first hashed
+    row (the documented legacy prefix from before migration 0002). A NULL
+    hash AFTER the chain has started is tampering — an attacker erasing the
+    hash of an intermediate row to hide an edit — and fails verification at
+    that row's id.
     """
     prev_hash: str | None = None  # None = still in legacy prefix
+    chain_started = False
     for entry in sess.query(AuditEntry).order_by(AuditEntry.id).all():
         if entry.entry_hash is None:
-            continue  # legacy row: outside the chain by documented genesis
+            if chain_started:
+                # tampering: a hash was erased in the middle of the chain
+                return False, entry.id
+            continue  # legacy prefix row: outside the chain by genesis
+        chain_started = True
         if prev_hash is None:
             expected_prev = "GENESIS"  # first hashed row anchors the chain
         else: