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>

internal/store/domains.go

@@ -16,9 +16,10 @@ var ErrDomainExists = errors.New("domain already exists")
 // ErrDomainNotFound is returned when a domain id/name does not exist.
 var ErrDomainNotFound = errors.New("domain not found")
 
-// Domain is a sending domain managed through the panel (spec 4.1). The DKIM key
-// material itself lives on disk under /data; this row records the selector and
-// metadata. AppCount is populated by the listing queries, not stored.
+// Domain is a sending domain managed through the panel (product.md §
+// Multi-domain model). The DKIM key material itself lives on disk under /data;
+// this row records the selector and metadata. AppCount is populated by the
+// listing queries, not stored.
 type Domain struct {
 	ID           int64
 	Name         string
@@ -28,7 +29,7 @@ type Domain struct {
 }
 
 // AddDomain inserts a new sending domain. The caller is responsible for having
-// validated name (spec 7.6.2) before it reaches SQL; the query is parameterised
+// validated name (security.md) before it reaches SQL; the query is parameterised
 // regardless. A duplicate name maps to ErrDomainExists.
 func (s *Store) AddDomain(name, selector string) (Domain, error) {
 	now := time.Now().UTC()
@@ -49,7 +50,7 @@ func (s *Store) AddDomain(name, selector string) (Domain, error) {
 	return Domain{ID: id, Name: name, DKIMSelector: selector, CreatedAt: now}, nil
 }
 
-// ListDomains returns every domain with its bound-application count (spec 7.2.2),
+// ListDomains returns every domain with its bound-application count (product.md),
 // ordered by name.
 func (s *Store) ListDomains() ([]Domain, error) {
 	rows, err := s.db.Query(`
@@ -92,7 +93,7 @@ func (s *Store) GetDomain(id int64) (Domain, error) {
 }
 
 // DeleteDomain removes a domain. Its applications and their address/binding rows
-// go with it via ON DELETE CASCADE (spec 7.2.4). Returns ErrDomainNotFound if no
+// go with it via ON DELETE CASCADE (product.md). Returns ErrDomainNotFound if no
 // such row existed.
 func (s *Store) DeleteDomain(id int64) error {
 	res, err := s.db.Exec("DELETE FROM domains WHERE id = ?", id)