feat(admin): module skeleton, config and two database connections

Adds the admin module: env config with fail-fast validation, two MongoDB
connections (its own vantage_admin database plus a narrow path into the
control plane), the boot sequence and the image.

Config refuses to start without a signing key, and both Mongo URIs must
name their database inline -- admin talks to two databases, so a bare
MONGO_DB would be ambiguous about which.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/cmd/main.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/joho/godotenv"
+	"github.com/mrhid6/vantage/admin/internal/config"
+	"github.com/mrhid6/vantage/admin/internal/db"
+)
+
+func main() {
+	godotenv.Load()
+
+	cfg, err := config.Load()
+	if err != nil {
+		log.Fatalf("configuration error: %v", err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second)
+	if err := db.Connect(ctx, cfg); err != nil {
+		cancel()
+		log.Fatalf("database: %v", err)
+	}
+	cancel()
+	log.Printf("connected: admin=%s control=%s", cfg.AdminDBName, cfg.ControlDBName)
+
+	srv := &http.Server{
+		Addr:              cfg.Addr,
+		Handler:           http.NotFoundHandler(), // replaced in Task 8
+		ReadHeaderTimeout: 10 * time.Second,
+		ReadTimeout:       20 * time.Second,
+		WriteTimeout:      30 * time.Second,
+		IdleTimeout:       60 * time.Second,
+	}
+
+	go func() {
+		log.Printf("admin listening on %s", cfg.Addr)
+		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
+			log.Fatalf("server error: %v", err)
+		}
+	}()
+
+	stopCtx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+	<-stopCtx.Done()
+
+	shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 15*time.Second)
+	defer shutdownCancel()
+	_ = srv.Shutdown(shutdownCtx)
+	log.Println("admin stopped")
+	os.Exit(0)
+}