fix(deploy): require every credential, and give sandbox its own domain

Three things review caught, and one shape correction.

**No credential has a default any more.** The compose file shipped
`ADMIN_SHARED_SECRET` falling back to a value written in this repository —
and that header bypasses token verification entirely, so anyone reading
the file could act as an operator against any deployment that had not
overridden it. A default is worth less than it looks here: the deployment
that never set the variable is exactly the one where the default is public.
Every credential now comes from `.env`, and compose refuses to start naming
the variable it wanted. That also takes the last literal password out of a
tracked file.

**The origin ports are on loopback.** Both services speak plain HTTP and
mark no cookie `Secure`, because both expect to sit behind something that
terminates TLS. Published on every interface they were a way to reach the
issuer around that proxy, with sign-in codes and tokens in clear text.

**Mail settings are passed through rather than fixed.** The issuer was
pinned to printing sign-in codes to its log, and the three delivery
settings never reached it — so the documented way to configure mail could
not work, and every code and recipient went to the container log instead.
Printing codes is now asked for in `.env` like everything else, and with
nothing configured the issuer refuses to send rather than logging.

**Sandbox becomes a domain rather than a prefix.** `api.sandbox.nestri.io`
and `auth.sandbox.nestri.io`, because sandbox holds whatever is not
production and that set grows. One certificate for `*.sandbox.nestri.io`
then covers all of it, including unpredictable per-pull-request names,
and cannot be presented for production's own domain — which the zone-wide
wildcard the previous shape leaned on could.

Also drops `STEAM_API_KEY`. It was declared in two type definitions and
read by nothing: linking an account makes no outbound call that needs it.

README.md

@@ -114,6 +114,7 @@ sandboxes, one GPU" possible instead of one tenant per card.
 
 ```sh
 bun install
+cp .env.example .env         # compose reads every credential from here
 docker compose up postgres   # the database
 bun run db:migrate           # schema
 bun dev                      # control plane, local Cloudflare runtime