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/domain/dkim.go

@@ -1,8 +1,8 @@
 // Package domain owns SelfPost's sending-domain model: per-domain DKIM key
 // generation, the OpenDKIM KeyTable/SigningTable that drive signing, and the
 // orchestration that keeps the SQLite registry, the on-disk keys and OpenDKIM
-// in agreement (spec 4.1, 6). Key material lives under /data so it survives
-// container restarts (spec 6.1, 9).
+// in agreement (architecture.md § OpenDKIM). Key material lives under /data so
+// it survives container restarts (architecture.md § OpenDKIM).
 package domain
 
 import (
@@ -46,7 +46,7 @@ func writePrivateKeyPEM(path string, key *rsa.PrivateKey) error {
 
 // loadPrivateKeyPEM reads and parses a PKCS#1 RSA private key written by
 // writePrivateKeyPEM. It is used to recompute the public DNS record on demand,
-// keeping the private key file the single source of truth (spec 7.2.10).
+// keeping the private key file the single source of truth (product.md).
 func loadPrivateKeyPEM(path string) (*rsa.PrivateKey, error) {
 	data, err := os.ReadFile(path)
 	if err != nil {
@@ -63,7 +63,7 @@ func loadPrivateKeyPEM(path string) (*rsa.PrivateKey, error) {
 	return key, nil
 }
 
-// DKIMRecord is the DNS TXT record a user must publish for a domain (spec 7.2.10).
+// DKIMRecord is the DNS TXT record a user must publish for a domain (product.md).
 type DKIMRecord struct {
 	// Name is the record's host, e.g. "selfpost._domainkey.example.com".
 	Name string
@@ -89,7 +89,7 @@ func dkimRecord(selector, domainName string, pub *rsa.PublicKey) (DKIMRecord, er
 // writeFileAtomic writes data to path via a temp file in the same directory
 // followed by a rename, so readers only ever see the complete old or new file.
 // It is the single safe-write primitive for DKIM keys and OpenDKIM tables
-// (spec 7.6.4).
+// (security.md).
 func writeFileAtomic(path string, data []byte, perm os.FileMode) error {
 	dir := filepath.Dir(path)
 	tmp, err := os.CreateTemp(dir, ".tmp-*")