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)

castle-api/src/castle_api/deploy_routes.py

@@ -0,0 +1,49 @@
+"""Deploy API endpoint."""
+
+from __future__ import annotations
+
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+
+from castle_core.deploy import deploy
+
+router = APIRouter(tags=["deploy"])
+
+
+class DeployRequest(BaseModel):
+    """Optional request body for deploy."""
+
+    name: str | None = None
+
+
+class DeployResponse(BaseModel):
+    """Response from a deploy operation."""
+
+    status: str
+    deployed_count: int
+    messages: list[str]
+
+
+@router.post("/deploy", response_model=DeployResponse)
+def run_deploy(request: DeployRequest | None = None) -> DeployResponse:
+    """Deploy services and jobs from castle.yaml to runtime.
+
+    Resolves env vars and secrets, generates systemd units and Caddyfile,
+    copies frontend build outputs, and reloads systemd.
+
+    Optionally pass a name to deploy a single service or job.
+    """
+    target_name = request.name if request else None
+
+    try:
+        result = deploy(target_name=target_name)
+    except FileNotFoundError as e:
+        raise HTTPException(status_code=404, detail=str(e)) from e
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=str(e)) from e
+
+    return DeployResponse(
+        status="ok",
+        deployed_count=result.deployed_count,
+        messages=result.messages,
+    )