fix: root-anchor mcables ignore pattern, commit cmd/mcables/main.go

The bare `mcables` pattern in .gitignore (line 11) and .dockerignore
(line 18) was intended to ignore the built binary at the repo root, but
without a leading slash it also matched the cmd/mcables/ directory. The
result: cmd/mcables/main.go was never tracked in git, and fresh worktrees
had to copy it from a sibling to build.

- Change `mcables` → `/mcables` in both files (still ignores the root
  binary; no longer matches the cmd subdirectory).
- Add cmd/mcables/main.go (copied from picasso's worktree, verified
  identical to head's main checkout).

Verified: `git check-ignore cmd/mcables/main.go` returns not-ignored;
a touched `./mcables` at the repo root is still ignored via `/mcables`.
`go build ./...` clean.

.dockerignore

@@ -15,7 +15,7 @@ data
 
 # Build artefacts
 bin
-mcables
+/mcables
 
 # Editor cruft
 .vscode

.gitignore

@@ -8,7 +8,7 @@ data/*.db-shm
 
 # Build artefacts
 bin/
-mcables
+/mcables
 
 # Editor
 .vscode/

cmd/mcables/main.go

@@ -0,0 +1,64 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"path/filepath"
+	"syscall"
+	"time"
+
+	"mgit.msbls.de/m/mcables/internal/db"
+	"mgit.msbls.de/m/mcables/internal/server"
+	"mgit.msbls.de/m/mcables/web"
+)
+
+func main() {
+	addr := envOr("MCABLES_ADDR", "0.0.0.0:7777")
+	dbPath := envOr("MCABLES_DB", "./data/mcables.db")
+
+	if err := os.MkdirAll(filepath.Dir(dbPath), 0o755); err != nil {
+		log.Fatalf("mkdir data dir: %v", err)
+	}
+
+	store, err := db.Open(dbPath)
+	if err != nil {
+		log.Fatalf("open db: %v", err)
+	}
+	defer store.Close()
+
+	if err := db.Migrate(store.DB()); err != nil {
+		log.Fatalf("migrate: %v", err)
+	}
+
+	srv := &http.Server{
+		Addr:              addr,
+		Handler:           server.New(store, web.Static()),
+		ReadHeaderTimeout: 5 * time.Second,
+	}
+
+	go func() {
+		log.Printf("mcables listening on %s (db=%s)", addr, dbPath)
+		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
+			log.Fatalf("listen: %v", err)
+		}
+	}()
+
+	stop := make(chan os.Signal, 1)
+	signal.Notify(stop, os.Interrupt, syscall.SIGTERM)
+	<-stop
+	log.Printf("shutting down")
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	_ = srv.Shutdown(ctx)
+}
+
+func envOr(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}