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>

internal/webserver/routes.go

@@ -188,7 +188,7 @@ func (ws *WebServer) handleLogin(w http.ResponseWriter, r *http.Request) {
 			Path:     "/",
 			HttpOnly: true,
 			Secure:   true,
-			SameSite: http.SameSiteLaxMode,
+			SameSite: http.SameSiteStrictMode,
 		})
 		http.Redirect(w, r, "/dashboard", http.StatusFound)
 	default: