Add MEK rotation, per-engine DEKs, and v2 ciphertext format (audit #6, #22)

Implement a two-level key hierarchy: the MEK now wraps per-engine DEKs
stored in a new barrier_keys table, rather than encrypting all barrier
entries directly. A v2 ciphertext format (0x02) embeds the key ID so the
barrier can resolve which DEK to use on decryption. v1 ciphertext remains
supported for backward compatibility.

Key changes:
- crypto: EncryptV2/DecryptV2/ExtractKeyID for v2 ciphertext with key IDs
- barrier: key registry (CreateKey, RotateKey, ListKeys, MigrateToV2, ReWrapKeys)
- seal: RotateMEK re-wraps DEKs without re-encrypting data
- engine: Mount auto-creates per-engine DEK
- REST + gRPC: barrier/keys, barrier/rotate-mek, barrier/rotate-key, barrier/migrate
- proto: BarrierService (v1 + v2) with ListKeys, RotateMEK, RotateKey, Migrate
- db: migration v2 adds barrier_keys table

Also includes: security audit report, CSRF protection, engine design specs
(sshca, transit, user), path-bound AAD migration tool, policy engine
enhancements, and ARCHITECTURE.md updates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/templates/pki.html

@@ -40,6 +40,7 @@
     <div class="card-title">Import Root CA</div>
     <p>{{if .RootExpired}}The current root CA has expired. Import a new one to continue issuing certificates.{{else}}No root CA is present. Import one to get started.{{end}}</p>
     <form method="post" action="/pki/import-root" enctype="multipart/form-data">
+        {{csrfField}}
         <div class="form-row">
             <div class="form-group">
                 <label for="cert_file">Certificate PEM file</label>
@@ -99,6 +100,7 @@
 <div class="card">
     <div class="card-title">Issue Certificate</div>
     <form method="post" action="/pki/issue">
+        {{csrfField}}
         <div class="form-row">
             <div class="form-group">
                 <label for="issue_cn">Common Name</label>
@@ -191,6 +193,7 @@
     </div>
     {{else}}
     <form method="post" action="/pki/sign-csr">
+        {{csrfField}}
         <div class="form-row">
             <div class="form-group">
                 <label for="sign_issuer">Issuer</label>
@@ -228,6 +231,7 @@
     <div class="card-title">Create Issuer</div>
     {{if .IssuerError}}<div class="error">{{.IssuerError}}</div>{{end}}
     <form method="post" action="/pki/create-issuer">
+        {{csrfField}}
         <div class="form-row">
             <div class="form-group">
                 <label for="issuer_name">Issuer Name</label>