Fix SSH encryption and add file download functionality

- Fix SSH authentication by ensuring all database operations use EncryptedDBOperations for automatic encryption/decryption
- Resolve SSH connection failures caused by encrypted password data being passed to authentication
- Add comprehensive file download functionality for SSH file manager (Issue #228)
- Update database migration to add require_password column for SSH sessions
- Enhance debugging and logging for SSH connection troubleshooting

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/backend/database/db/index.ts

@@ -250,6 +250,12 @@ const migrateSchema = () => {
     "INTEGER REFERENCES ssh_credentials(id)",
   );
 
+  addColumnIfNotExists(
+    "ssh_data",
+    "require_password",
+    "INTEGER NOT NULL DEFAULT 1",
+  );
+
   // SSH credentials table migrations for encryption support
   addColumnIfNotExists("ssh_credentials", "private_key", "TEXT");
   addColumnIfNotExists("ssh_credentials", "public_key", "TEXT");