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

go.mod

@@ -0,0 +1,14 @@
+module git.kessinen.com/kessinen/levyraati26-go
+
+go 1.24
+
+require github.com/jackc/pgx/v5 v5.7.2
+
+require (
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	golang.org/x/crypto v0.32.0 // indirect
+	golang.org/x/sync v0.10.0 // indirect
+	golang.org/x/text v0.21.0 // indirect
+)