refactor(crypto): remove pending share queue and credential sharing key

With server-unwrappable DEKs both sides of a share are always available,
so the needsReEncryption queue, CREDENTIAL_SHARING_KEY and the system_*
shadow columns on ssh_credentials are gone. A one-time boot cleanup
re-creates legacy pending share copies where possible (dropping
unresolvable ones with a warning) and drops the legacy columns.

src/backend/starter.ts

@@ -124,6 +124,10 @@ import {
       await import("./utils/crypto-migration/dek-migration.js");
     await runBootDekMigration({ cleanupLegacy: true });
 
+    const { runLegacySharedCredentialCleanup } =
+      await import("./utils/crypto-migration/legacy-share-cleanup.js");
+    await runLegacySharedCredentialCleanup();
+
     const authManager = AuthManager.getInstance();
     await authManager.initialize();
     DataCrypto.initialize();