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>

test/e2e/panel_client.go

@@ -145,7 +145,7 @@ func (c *panelClient) dkimRecord(domainID string) (name, value string, err error
 }
 
 // addApplication submits the add-application form and returns the one-shot
-// login/password the panel renders inline (spec 7.6.1 — never recoverable
+// login/password the panel renders inline (security.md — never recoverable
 // later, so this is the only place to read it).
 func (c *panelClient) addApplication(domainID, login, mode, addresses string) (appLogin, password string, err error) {
 	resp, body, err := c.postForm("/domains/"+domainID+"/applications", url.Values{
@@ -170,8 +170,9 @@ func (c *panelClient) addApplication(domainID, login, mode, addresses string) (a
 	return appLogin, password, nil
 }
 
-// setRateLimit saves a level-2 differentiated limit (spec 7.4) on either a
-// domain (/domains/{id}/ratelimit) or an application (/applications/{id}/ratelimit).
+// setRateLimit saves a level-2 differentiated limit (README § Rate limiting)
+// on either a domain (/domains/{id}/ratelimit) or an application
+// (/applications/{id}/ratelimit).
 func (c *panelClient) setRateLimit(path, allowedIP string, maxMessages, windowSeconds int) error {
 	resp, body, err := c.postForm(path, url.Values{
 		"allowed_ips":    {allowedIP},