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>

cmd/selfpost-backup/main.go

@@ -1,6 +1,6 @@
 // Command selfpost-backup produces the full persistent-state archive from inside
 // the container, invoked via `docker exec` for scripted/cron backups — the CLI
-// equivalent of the panel's backup button (spec 7.5.A, 11.6).
+// equivalent of the panel's backup button (architecture.md § Persistence).
 //
 // By default the gzip-compressed tar is written to stdout, so the usual form is:
 //
@@ -8,7 +8,7 @@
 //
 // Use -o to write to a file instead. The resulting archive contains DKIM private
 // keys, the admin password hash and SASL credentials — treat it as a secret
-// (spec 7.5.A).
+// (architecture.md § Persistence).
 //
 // Given a password (SELFPOST_BACKUP_PASSWORD or -password-file, never an
 // argument, which would show up in the process list) the archive is written as
@@ -26,9 +26,9 @@ import (
 	"path/filepath"
 	"strings"
 
-	"codeberg.org/mix/selfpost/internal/backup"
-	"codeberg.org/mix/selfpost/internal/buildinfo"
-	"codeberg.org/mix/selfpost/internal/secretfile"
+	"github.com/mixeme/selfpost/internal/backup"
+	"github.com/mixeme/selfpost/internal/buildinfo"
+	"github.com/mixeme/selfpost/internal/secretfile"
 )
 
 // passwordEnv names the environment variable holding the encryption password.