feat: build-on-node — git checkouts, remote image builds, image transport (#60)

Git-sourced deploys never materialized a checkout: sourcePath was
build_path resolved against the server cwd, so builds consumed the
backend's own directory. Remote placement could only run
registry-pulled images.

- internal/source: per-deployment checkout (init/fetch/checkout) at
  branch or commit; GIT_ASKPASS keeps tokens out of argv, .git/config,
  and the build context; dumb-transport and SHA-refusal fallbacks;
  build_path validated inside the checkout.
- BuildManager.PackageContext: produces the same tar context Build
  ships to the daemon — generated Dockerfile at tar root, .git
  excluded. Fixes nixpacks passing the Dockerfile path as content.
- Agent artifact endpoint: GET /api/agents/artifacts/:id streams
  build contexts and docker-save tarballs behind agent-token auth;
  hex-validated ids, per-deployment cleanup, boot-time purge.
- Agent commands: build_image (docker build -, --build-arg values via
  process env, not argv) and load_image (docker load for containr-*
  rollback tags no registry serves).
- Deployment flow: remote git builds package context → artifact →
  node docker-builds the deployment tag; remote rollbacks of local
  tags export via SaveImage → artifact → node docker-loads.

Verified end-to-end: agent-pinned git service built on the node
(build_image → create_container → deployed) and rollback shipped a
docker-save tarball (load_image → deployed). Artifact auth returns
401/404 correctly; workspaces and blobs are removed after deploy.

Authored By: TDvorak <[email protected]>

app/backend/internal/api/routes.go

@@ -5,6 +5,7 @@ import (
 	"log"
 	"net/http"
 	"os"
+	"path/filepath"
 	"time"
 
 	"containr/internal/build"
@@ -43,6 +44,8 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 		dockerClient = client
 		buildManager = build.NewBuildManager("/tmp/containr-builds", dockerClient)
 		deploymentEngine = deployment.NewDeploymentEngine(buildManager, dockerClient)
+		initArtifacts(filepath.Join("/tmp/containr-builds", "artifacts"))
+		purgeStaleArtifacts()
 		// Node-pinned services dispatch container lifecycle to the agent's
 		// command queue instead of the local Docker socket.
 		deploymentEngine.SetNodeRunner(newAgentNodeRunner(db))