feat(crypto): boot-time DEK migration to system-wrapped v3 format

utils/crypto-migration/dek-migration.ts carries the legacy unwrap paths
(PBKDF2 password KEK, OIDC/WebAuthn system keys, hardcoded-default
fallback) and migrates every server-unwrappable DEK to the v3 wrap at
startup. Password-wrapped DEKs migrate at next login or from a live
session via adoptRecoveredDEK. Legacy rows are kept for now; cleanup
flips on once the new path is authoritative.

src/backend/starter.ts

@@ -117,6 +117,13 @@ import {
       operation: "backend_init_db",
     });
 
+    const { UserKeyManager } = await import("./utils/user-keys.js");
+    await UserKeyManager.getInstance().initialize();
+
+    const { runBootDekMigration } =
+      await import("./utils/crypto-migration/dek-migration.js");
+    await runBootDekMigration();
+
     const authManager = AuthManager.getInstance();
     await authManager.initialize();
     DataCrypto.initialize();