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/db/migrate.go

@@ -30,6 +30,15 @@ var migrations = []string{
 		version INTEGER PRIMARY KEY,
 		applied_at DATETIME NOT NULL DEFAULT (datetime('now'))
 	);`,
+
+	// Version 2: barrier key registry for per-engine DEKs
+	`CREATE TABLE IF NOT EXISTS barrier_keys (
+		key_id       TEXT PRIMARY KEY,
+		version      INTEGER NOT NULL DEFAULT 1,
+		encrypted_dek BLOB NOT NULL,
+		created_at   DATETIME NOT NULL DEFAULT (datetime('now')),
+		rotated_at   DATETIME NOT NULL DEFAULT (datetime('now'))
+	);`,
 }
 
 // Migrate applies all pending migrations.