fix(audit): frame hash inputs with TLV (#7492)

Hash new audit entries from a TLV buffer built with one-byte field tags,
big-endian u64 byte lengths, and raw values. Absent optional fields are
omitted; present empty values retain their tag and zero length. This
prevents bytes from moving between fields without restricting object IDs
or inferring boundaries from field contents.

Store an explicit `hash_version`. Migration 0048 labels existing rows as
version 1, and new writers use version 2. Historical hashes are
unchanged and mixed-version chains verify. The legacy encoding retains
its original ambiguity; TLV protects new entries. Apply the migration
before deploying the updated writers and verifiers.

Tests cover both boundary collisions, absent versus empty fields, stable
digests, unknown versions, mixed-version chains, and stored-row
tampering. All 16 audit and 128 database unit tests passed. The six
PostgreSQL audit tests passed against both migrated and fresh schemas,
and a populated-row migration check preserved the historical hash. Full
`just ci` passed.

Related audit fixes: #4216 and #4249; issue #4173.

Signed-off-by: Jordan Mecom <jm@squareup.com>

Justfile

@@ -365,6 +365,7 @@ test-unit:
     ./scripts/test-ensure-local-relay-key.sh
     if command -v cargo-nextest &>/dev/null; then
         cargo nextest run -p buzz-core -p buzz-auth --lib
+        cargo nextest run -p buzz-audit --lib
         # buzz-auth NIP-FI verifier doctests. The sealed-authority
         # `compile_fail` doctests prove the default-feature public API alone
         # cannot forge the issuer→JWKS authority; nextest does not run