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

.env.example

@@ -0,0 +1,7 @@
+# Copy to .env and edit. Neither password has a default.
+POSTGRES_PASSWORD=
+ADMIN_USER=admin
+ADMIN_PASSWORD=
+
+# Set to false only for local development over plain HTTP.
+SECURE_COOKIES=true