feat(templates): git compose import, six stack seeds, empty-default vars (#63)

* feat(templates): git compose import, six stack seeds, empty-default vars

- POST /templates/import/git fetches a compose file from any git remote
  (clone URL, ssh, owner/repo via connected providers — private repos
  included), ref selects branch or commit. Path defaults through
  compose.yaml/yml then docker-compose.yml/yaml. Reuses the build-on-node
  checkout machinery; repo-relative paths are containment-checked.
- CLI `templates import-git <repo> [path] [--ref]`, MCP
  containr_templates_import_git, OpenAPI entry, and a paste/git toggle
  in the template import dialog.
- {{VAR:-}} with an empty default now resolves instead of reporting the
  var missing — matches shell :- semantics.
- SeedTemplates gains six official graph stacks — n8n (pg+redis+worker),
  Gitea, Nextcloud, Vaultwarden, Umami, Immich — wired with
  {{service.*}} refs and {{secret}} credentials; no hardcoded passwords.
- TestSeedTemplatesResolveCleanly parses, topo-sorts, validates, and
  resolves every seeded graph — catalog drift can't merge silently.
- templates/n8n.md compose block had invalid YAML (mixed depends_on
  list/map) — fixed so the documented stack actually parses.

Verified live: import/git on a dumb-HTTP repo returns the converted
graph (db image → managed postgresql), path/ref/traversal/unauth edges
return clean errors, tpl-n8n plan resolves with zero unresolved vars.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* ci: pre-pull compose-smoke images via gcr mirror

Anonymous Docker Hub pulls rate-limit shared CI egress IPs — the compose
boot smoke flaked on traefik:pull. postgres/traefik are library/* images
served by mirror.gcr.io without auth; compose reuses the retagged local
images, falling back to a normal pull when the mirror misses.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* ci: route docker.io through gcr mirror for compose smoke

Pull-time pre-pulls don't cover BuildKit's base-image metadata lookups
(node:22-alpine, nginx:alpine, golang:1.26-alpine all 429). Daemon-level
registry-mirrors covers pulls and resolution in one move.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* ci: mirror docker.io in buildx for image publish job

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* ci: route smoke pulls/builds through mirror.gcr.io via compose overlay

daemon.json registry-mirrors isn't honored by the runner's containerd
image store. Qualify refs directly: ARG-parameterized Dockerfile FROMs
(defaults unchanged) plus a CI-only compose overlay that points image
pulls and build args at mirror.gcr.io.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/api/templates_test.go

@@ -1,6 +1,9 @@
 package api
 
-import "testing"
+import (
+	"encoding/json"
+	"testing"
+)
 
 func TestNormalizeTemplateServiceType(t *testing.T) {
 	valid := []string{"web", "worker", "database", "cron"}
@@ -71,3 +74,32 @@ func TestResolveTemplateRuntimeImage(t *testing.T) {
 		t.Fatalf("expected unknown runtime passthrough, got %q", got)
 	}
 }
+
+func TestSeedTemplatesResolveCleanly(t *testing.T) {
+	for _, tpl := range SeedTemplates() {
+		var config TemplateConfig
+		if err := json.Unmarshal([]byte(tpl.Config), &config); err != nil {
+			t.Fatalf("%s: invalid config JSON: %v", tpl.ID, err)
+		}
+		var variables []TemplateVariable
+		if err := json.Unmarshal([]byte(tpl.Variables), &variables); err != nil {
+			t.Fatalf("%s: invalid variables JSON: %v", tpl.ID, err)
+		}
+		if len(config.Services) == 0 {
+			continue // legacy single-service template
+		}
+		sorted, err := topoSortServices(config.Services)
+		if err != nil {
+			t.Fatalf("%s: topo sort: %v", tpl.ID, err)
+		}
+		if err := validateTemplateGraph(sorted); err != nil {
+			t.Fatalf("%s: graph validation: %v", tpl.ID, err)
+		}
+		// User vars left empty — every expression must still resolve via
+		// defaults, secrets, or service refs.
+		graph := resolveTemplateGraph(sorted, map[string]string{})
+		if len(graph.unresolved) > 0 {
+			t.Fatalf("%s: unresolved expressions: %v", tpl.ID, graph.unresolved)
+		}
+	}
+}