refactor: Extract deploy logic to castle-core and add deploy API endpoint

- Extract all deploy logic from CLI into castle-core/deploy.py as a reusable deploy() function
  * Resolves services/jobs to DeployedComponents
  * Generates systemd units and Caddyfile
  * Copies frontend assets and reloads systemd
  * Returns DeployResult with deployed count and messages
- Simplify CLI deploy command to a thin wrapper calling castle_core.deploy()
- Add POST /deploy endpoint to castle-api for remote deployment management
  * Supports optional {name} body param to deploy one or all services
  * Returns deployment result as JSON
- Register deploy router in castle-api main.py
- Update README with new deploy endpoint documentation

This enables full remote management: build, test, lint, and deploy any castle
node over HTTP (e.g. POST http://node:9000/api/deploy)

README.md

@@ -196,6 +196,8 @@ When enabled, the API publishes the node's registry to `castle/{hostname}/regist
 | `GET /gateway/caddyfile` | Generated Caddyfile content |
 | `POST /gateway/reload` | Regenerate Caddyfile and reload Caddy |
 | `GET /status` | Live health for all services |
+| **Deploy** | |
+| `POST /deploy` | Deploy all services and jobs (spec to runtime) |
 | **Config** | |
 | `GET /config` | Read castle.yaml |
 | `PUT /config` | Write castle.yaml |