docs: D6-D9 — HEALTHCHECK, env regression test, new docs, archive spec

Add Docker HEALTHCHECK and mail-path /healthz liveness; env-doc regression
test; architecture.md and development.md; product.md and expanded security.md;
retire live specification.md to docs/archive/.

Co-Authored-By: Claude <claude-opus-5-thinking-high@noreply@anthropic.com>

docs/product.md

@@ -0,0 +1,102 @@
+# SelfPost — product boundaries
+
+**What this file is.** Stable product definition for SelfPost v1.0: purpose,
+deployment assumptions, explicit out-of-scope items, and the multi-domain
+model. User-facing install and operations live in [README.md](../README.md);
+as-built technical detail in [architecture.md](architecture.md).
+
+---
+
+## Purpose
+
+SelfPost is a self-hosted **outbound SMTP relay** with a web control panel,
+shipped as a single Docker image. It sends mail directly to the internet from
+your own IP with per-domain DKIM signing.
+
+**Primary workflow:** configure the relay once (domains, DKIM, SASL
+applications), then point scripts and apps at the SMTP endpoint. SelfPost
+delivers as the configured sending domain.
+
+---
+
+## Deployment context (fixed assumptions)
+
+These constraints are intentional; changing them requires an explicit product
+decision:
+
+1. **VPS or home server** — one image for both. Raspberry Pi and similar boards
+   are not a current target but not ruled out forever.
+2. **Send from your own IP (DIY)** — no intermediate relay provider. Requires
+   outbound port 25, static IP, and configurable PTR/rDNS from the operator.
+3. **Single container** — Postfix, OpenDKIM, and the panel run under one
+   `supervisord` inside one image.
+4. **Panel is internet-facing** — mandatory security requirements in
+   [security.md](security.md).
+
+**Infrastructure the operator provides (not SelfPost features):** unblocked
+outbound TCP 25, static IP, PTR/rDNS, acceptable IP reputation. SelfPost does
+not detect, bypass, or compensate for missing prerequisites; mail simply fails
+to deliver when they are absent.
+
+---
+
+## Out of scope
+
+Explicitly excluded to prevent scope creep:
+
+- Inbound mail (IMAP/POP3, mailboxes, delivery to user inboxes)
+- Webmail
+- Multi-user panel / organisations / roles — one administrator; managing
+  **multiple sending domains** is in scope (see below)
+- Inbound antispam/antivirus (rspamd, ClamAV, etc.)
+- A custom MTA — Postfix is used as-is
+- Dovecot or a full mail stack for SASL — Cyrus SASL (`sasldb2`) only
+
+Future line **2.x.x** (optional inbound relay, domain-admin role) is tracked in
+[roadmap.md](roadmap.md) and requires explicit approval before implementation.
+
+---
+
+## Multi-domain model
+
+SelfPost is a **multi-domain outbound relay**. Two linked entities:
+
+### Sending domain
+
+Example: `example.com`. Has its own DKIM key and selector.
+
+### Application (account)
+
+A SASL login/password bound to **one** domain. A domain may have several
+applications (e.g. newsletter vs alerts), each with its own credentials, but
+each may send **only from its own domain**, never from another.
+
+### From-address mode (per application)
+
+Set when creating or editing an application:
+
+1. **Any address in the domain** — `*@example.com` (wildcard in
+   `smtpd_sender_login_maps`).
+2. **Explicit address list** — only listed From addresses within the domain;
+   anything else is rejected even if it belongs to the same domain.
+
+In both modes the From address must belong to the application's domain.
+
+### What the panel manages per domain
+
+- DKIM key + selector (shared by all applications on that domain)
+- One or more applications (SASL credentials + From mode + optional rate limits)
+- DNS guidance (DKIM TXT; reminders for SPF/DMARC)
+
+Adding a domain does **not** create an application automatically.
+
+### Lifecycle
+
+- **Add domain** — record, DKIM key, DNS instructions; no application yet.
+- **Add application** — SASL pair (password shown once), From mode, map entries,
+  `postfix reload`.
+- **Delete domain** — removes DKIM key and **all** its applications.
+- **Delete application** — removes only that app's SASL and map entries.
+
+This is not multi-tenancy (one admin); it is one owner operating several
+sending domains with independent application credentials.