Add skeleton: config, migrations, startup sweep, two listeners

Step 1 of the build order in docs/decisions.md. Boots, applies migrations
before serving, and serves a health check and an empty admin page.

- 001_init.sql is the full schema from docs/spec.md, including the check
  constraints and indexes the old app lacked
- The startup sweep fails submissions left mid-conversion by a restart; an
  in-process goroutine dies with the process and those rows would otherwise
  say converting forever
- Admin is Basic Auth from env on its own listener, fatal at startup when
  ADMIN_PASSWORD is unset

main_test.go

@@ -0,0 +1,75 @@
+package main
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func TestRequireAdmin(t *testing.T) {
+	a := &app{cfg: config{adminUser: "admin", adminPass: "s3cret"}}
+	h := a.requireAdmin(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusTeapot)
+	}))
+
+	for _, tc := range []struct {
+		name, user, pass string
+		auth             bool
+		want             int
+	}{
+		{name: "no credentials", want: http.StatusUnauthorized},
+		{name: "wrong password", user: "admin", pass: "hunter2", auth: true, want: http.StatusUnauthorized},
+		{name: "wrong user", user: "root", pass: "s3cret", auth: true, want: http.StatusUnauthorized},
+		{name: "correct", user: "admin", pass: "s3cret", auth: true, want: http.StatusTeapot},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			r := httptest.NewRequest("GET", "/admin", nil)
+			if tc.auth {
+				r.SetBasicAuth(tc.user, tc.pass)
+			}
+			w := httptest.NewRecorder()
+			h.ServeHTTP(w, r)
+			if w.Code != tc.want {
+				t.Fatalf("status = %d, want %d", w.Code, tc.want)
+			}
+		})
+	}
+}
+
+// Set TEST_DATABASE_URL to run this against a throwaway database.
+func TestMigrateIsIdempotent(t *testing.T) {
+	url := os.Getenv("TEST_DATABASE_URL")
+	if url == "" {
+		t.Skip("TEST_DATABASE_URL not set")
+	}
+	ctx := context.Background()
+	pool, err := pgxpool.New(ctx, url)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer pool.Close()
+
+	if _, err := pool.Exec(ctx, `drop schema public cascade; create schema public`); err != nil {
+		t.Fatal(err)
+	}
+	for i := range 2 {
+		if err := migrate(ctx, pool); err != nil {
+			t.Fatalf("migrate run %d: %v", i+1, err)
+		}
+	}
+	if err := sweep(ctx, pool); err != nil {
+		t.Fatalf("sweep: %v", err)
+	}
+
+	var n int
+	if err := pool.QueryRow(ctx, `select count(*) from schema_migrations`).Scan(&n); err != nil {
+		t.Fatal(err)
+	}
+	if n != 1 {
+		t.Fatalf("applied migrations = %d, want 1", n)
+	}
+}