feat: Sites feature backend

docs/configuration.md

@@ -10,7 +10,7 @@ Copy [`.env.example`](../.env.example) to `.env` to get started. When a required
 
 | Name            | Description                                                                                                         | Example                    |
 |-----------------|---------------------------------------------------------------------------------------------------------------------|----------------------------|
-| `PUBLIC_URL`    | Public base URL of the site, including scheme and no trailing slash. Used for links in emails and in the startup log. | `https://cms.example.com`  |
+| `PUBLIC_URL`    | Public base URL of the site, including scheme and no trailing slash. Used for links in emails and in the startup log. It is also the CMS's own address: at the first migration, when the sites table is empty, it seeds the default site (name "Default", key "default") with its scheme and host (path dropped, non-loopback `http` upgraded to `https`); an unparseable value stops the migration. It is never read for sites afterwards. | `https://cms.example.com`  |
 | `JWT_SECRET`    | Secret used to sign the auth JWT. Use a long random value; changing it logs everyone out.                            | `openssl rand -hex 32`     |
 | `SMTP_LOGIN`    | SMTP username used to authenticate with the server. Some providers (e.g. Brevo) issue a login distinct from the sender address. | `noreply@example.com` |
 | `SMTP_SENDER`   | Sender address used in the `From` header. Must be an address/domain the SMTP provider allows that login to send as.  | `noreply@example.com`      |