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/artifacts.go

@@ -0,0 +1,136 @@
+package api
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+// containrWorkDir is the build workspace root — same dir the BuildManager
+// was constructed with; checkouts and agent artifacts live beneath it.
+const containrWorkDir = "/tmp/containr-builds"
+
+// artifacts is a one-shot blob store for node agents: build contexts and
+// docker-save tarballs the server produced on behalf of a deployment. Files
+// live under workdir/artifacts/<id> and are removed when the deployment
+// finishes or after maxArtifactAge.
+var artifacts = &artifactStore{files: map[string]string{}}
+
+type artifactStore struct {
+	mu    sync.Mutex
+	dir   string
+	files map[string]string
+}
+
+const maxArtifactAge = 2 // hours — a deployment can never outlive this
+
+// initArtifacts points the store at dir (created on demand).
+func initArtifacts(dir string) {
+	artifacts.mu.Lock()
+	defer artifacts.mu.Unlock()
+	artifacts.dir = dir
+	_ = os.MkdirAll(dir, 0o755)
+}
+
+// artifactDir returns the store root, defaulting under the build workdir.
+func artifactDir() string {
+	artifacts.mu.Lock()
+	defer artifacts.mu.Unlock()
+	if artifacts.dir != "" {
+		return artifacts.dir
+	}
+	return filepath.Join(containrWorkDir, "artifacts")
+}
+
+// putArtifact streams rc into the store and returns the artifact id.
+func putArtifact(rc io.Reader) (string, error) {
+	buf := make([]byte, 16)
+	if _, err := rand.Read(buf); err != nil {
+		return "", err
+	}
+	id := hex.EncodeToString(buf)
+	dir := artifactDir()
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return "", err
+	}
+	path := filepath.Join(dir, id+".tar")
+	f, err := os.Create(path)
+	if err != nil {
+		return "", err
+	}
+	if _, err := io.Copy(f, rc); err != nil {
+		f.Close()
+		os.Remove(path)
+		return "", err
+	}
+	f.Close()
+
+	artifacts.mu.Lock()
+	artifacts.files[id] = path
+	artifacts.mu.Unlock()
+	return id, nil
+}
+
+// removeArtifact deletes the file and forgets the id. Safe on missing ids.
+func removeArtifact(id string) {
+	artifacts.mu.Lock()
+	path, ok := artifacts.files[id]
+	delete(artifacts.files, id)
+	artifacts.mu.Unlock()
+	if ok {
+		_ = os.Remove(path)
+		return
+	}
+	// Registered before a restart — remove best-effort by name.
+	if id != "" {
+		_ = os.Remove(filepath.Join(artifactDir(), id+".tar"))
+	}
+}
+
+// ServeArtifact streams a stored blob to a token-authenticated agent. Files
+// persist across downloads — spread builds share one context — and are
+// deleted by the deployment that created them.
+func (h *NodeAgentHandler) ServeArtifact(c *gin.Context) {
+	if !h.isValidAgentAuthToken(c.Request.Context(), agentAuthTokenFromRequest(c)) {
+		c.JSON(http.StatusUnauthorized, gin.H{"error": "Invalid auth token"})
+		return
+	}
+	id := c.Param("id")
+	raw, err := hex.DecodeString(id)
+	if len(id) != 32 || err != nil || len(raw) != 16 {
+		c.JSON(http.StatusNotFound, gin.H{"error": "Artifact not found"})
+		return
+	}
+	path := filepath.Join(artifactDir(), id+".tar")
+	info, err := os.Stat(path)
+	if err != nil || info.IsDir() {
+		c.JSON(http.StatusNotFound, gin.H{"error": "Artifact not found"})
+		return
+	}
+	c.Header("Content-Type", "application/x-tar")
+	c.File(path)
+}
+
+// purgeStaleArtifacts drops files older than maxArtifactAge. Called on boot.
+func purgeStaleArtifacts() {
+	entries, err := os.ReadDir(artifactDir())
+	if err != nil {
+		return
+	}
+	for _, e := range entries {
+		info, err := e.Info()
+		if err != nil {
+			continue
+		}
+		if info.ModTime().Before(time.Now().Add(-maxArtifactAge * time.Hour)) {
+			_ = os.Remove(filepath.Join(artifactDir(), e.Name()))
+		}
+	}
+}