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/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: