chore/docs: move to GitHub as the single home; drop archived-spec references

Codeberg is being retired as the project's public site, so every reference now
points at GitHub. That includes the Go module path (codeberg.org/mix/selfpost →
github.com/mixeme/selfpost): leaving an import path on a host that is going
away would break `go get` and `go install`, so this is not only a docs change.
Touches go.mod, test/e2e/go.mod, all imports, Makefile MODULE, the -ldflags
version stamp in build/Dockerfile and docs/development.md, the licence headers
in the SVG/HTML assets, and README (no more primary/mirror pair).

Comments no longer cite the archived specification. "spec 7.6.1", "spec 5.1"
and friends pointed into docs/archive/specification-v1.0.md, which is marked as
not a source of truth; each is now a reference to the live document that owns
the subject — architecture.md (with section), product.md, security.md or the
README. The review only asked for the 7.x refs (code-review.md § 4), but 4/5/6/
8/9 had the same defect, so they went too. Comments only, no behaviour change.

Also closes the remaining review items: architecture.md gained a Code layers
section with the layer diagram (A2), and TestParseDelivery gained the exotic
mail.log cases (§ 3).

Fixes a bug that last test found: the delivery-line pattern matched status=
greedily, taking the *last* occurrence on the line. Postfix appends the remote
server's reply verbatim, so a rejection whose reply quoted "status=sent" was
filed as a delivered message in the send log. It now takes the first status=
after the recipient, which is the real field.

R7 (CONTRIBUTING.md) moved to roadmap 2.x — one developer, no external PR flow,
so the file would have no audience yet. R1 (compose image tag) and the git tag
stay in roadmap § v1.x as the release-commit steps.

gofmt/go vet clean on both modules; go test ./... green except the three known
Windows-only failures (file perms, backslash paths, renaming an open file).
Not exercised on the dev server — no Docker locally.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/architecture.md

@@ -166,6 +166,67 @@ cookie and idle timeout has not expired.
 
 ---
 
+## Code layers
+
+Handlers never touch SQLite or the filesystem directly; every write that has to
+land in more than one place (SQLite row, `sasldb2` entry, Postfix map, OpenDKIM
+table) goes through a service, which is also where the rollback of a partial
+failure lives. The adapters below the services are the only code that knows
+about Postfix, OpenDKIM, DNS or the log file, which is what makes them
+substitutable in tests — `milter.Store`, `app.SenderMaps` and
+`logtail.StatusStore` are the seams the unit tests replace with fakes.
+
+```mermaid
+flowchart TB
+  subgraph cmd ["cmd — composition root"]
+    panel["panel: HTTP + journal-milter + log-tailer"]
+    backupcli["selfpost-backup CLI"]
+  end
+  subgraph web ["internal/web — HTTP surface"]
+    handlers["handlers_*.go, templates, session/security"]
+  end
+  subgraph services ["Services — multi-store operations + rollback"]
+    domainSvc["internal/domain"]
+    appSvc["internal/app"]
+  end
+  subgraph persistence ["Persistence"]
+    store["internal/store — SQLite, embedded migrations"]
+  end
+  subgraph adapters ["Adapters — the only infrastructure-aware code"]
+    postfix["internal/postfix"]
+    milterPkg["internal/milter"]
+    logtail["internal/logtail"]
+    dnscheck["internal/dnscheck"]
+    backupPkg["internal/backup"]
+    health["internal/health"]
+    secretfile["internal/secretfile"]
+  end
+  panel --> web
+  panel --> milterPkg
+  panel --> logtail
+  backupcli --> backupPkg
+  backupcli --> secretfile
+  web --> domainSvc
+  web --> appSvc
+  web --> backupPkg
+  web --> dnscheck
+  web --> health
+  web --> secretfile
+  domainSvc --> store
+  appSvc --> store
+  milterPkg --> store
+  logtail --> store
+  domainSvc --> postfix
+  appSvc --> postfix
+```
+
+The three roles inside the `panel` process (HTTP server, journal-milter,
+log-tailer goroutine) share one binary and one SQLite handle on purpose — see
+[Panel binary](#panel-binary-cmdpanel) for why, and *Persistence* below for the
+single-connection trade-off that follows from it.
+
+---
+
 ## Persistence (`/data` bind mount)
 
 | Path | Contents |