feat: Git-based backup system replacing JSON bundle backup/restore

DB is source of truth with git as downstream replica. SSH key generated
on first start, all resource mutations committed as apply-compatible YAML.
Supports manual commit import, conflict resolution (DB wins), disaster
recovery (empty DB restores from git), and timeline branches on restore.

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

stack/docker-compose.yml

@@ -32,11 +32,13 @@ services:
       MCPD_PYTHON_RUNNER_IMAGE: mysources.co.uk/michal/mcpctl-python-runner:latest
       MCPD_RATE_LIMIT_MAX: "500"
       MCPD_MCP_NETWORK: mcp-servers
+      MCPD_BACKUP_REPO: ${MCPD_BACKUP_REPO:-}
     depends_on:
       postgres:
         condition: service_healthy
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
+      - mcpctl-backup:/data/backup
     networks:
       - mcpctl
       - mcp-servers
@@ -58,3 +60,4 @@ networks:
 
 volumes:
   mcpctl-pgdata:
+  mcpctl-backup: