feat: store backup config in DB secret instead of env var

Move backup SSH keys and repo URL from MCPD_BACKUP_REPO env var to a
"backup-ssh" secret in the database. Keys are auto-generated on first
init and stored back into the secret. Also fix ERR_HTTP_HEADERS_SENT
crash caused by reply.send() without return in routes when onSend hook
is registered.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

stack/docker-compose.yml

@@ -32,7 +32,6 @@ services:
       MCPD_PYTHON_RUNNER_IMAGE: mysources.co.uk/michal/mcpctl-python-runner:latest
       MCPD_RATE_LIMIT_MAX: "2000"
       MCPD_MCP_NETWORK: mcp-servers
-      MCPD_BACKUP_REPO: ${MCPD_BACKUP_REPO:-}
     depends_on:
       postgres:
         condition: service_healthy