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/web/handlers_apps.go

@@ -7,13 +7,13 @@ import (
 	"strconv"
 	"strings"
 
-	"codeberg.org/mix/selfpost/internal/dnscheck"
-	"codeberg.org/mix/selfpost/internal/domain"
-	"codeberg.org/mix/selfpost/internal/store"
+	"github.com/mixeme/selfpost/internal/dnscheck"
+	"github.com/mixeme/selfpost/internal/domain"
+	"github.com/mixeme/selfpost/internal/store"
 )
 
 // newCred carries a freshly generated login/password to the template so it can
-// be shown exactly once (spec 7.6.1). It is never read back from storage.
+// be shown exactly once (security.md). It is never read back from storage.
 type newCred struct {
 	Login    string
 	Password string
@@ -29,8 +29,9 @@ type detailView struct {
 	FormMode  string
 	FormAddrs string
 	NewCred   *newCred
-	// RateLimitErr surfaces a validation error from a domain- or application-level
-	// rate-limit form (spec 7.4) as a page banner.
+	// RateLimitErr surfaces a validation error from a domain- or
+	// application-level rate-limit form (README § Rate limiting) as a page
+	// banner.
 	RateLimitErr string
 	// ExportErr surfaces a rejected encryption password from the export card.
 	ExportErr string
@@ -47,9 +48,9 @@ type appRateLimitView struct {
 	WindowVal string // window seconds, defaulted when unset
 }
 
-// handleDomainDetail shows a single domain: its DKIM DNS record (spec 7.2.10)
+// handleDomainDetail shows a single domain: its DKIM DNS record (product.md)
 // and its applications with the controls to add, edit, delete and re-issue
-// credentials (spec 7.2.5-9).
+// credentials (product.md).
 func (s *Server) handleDomainDetail(w http.ResponseWriter, r *http.Request) {
 	d, ok := s.lookupDomain(w, r)
 	if !ok {
@@ -214,7 +215,7 @@ func detailFlash(r *http.Request) string {
 }
 
 // handleAddApplication creates an application on a domain and renders the page
-// back with the generated password shown once (spec 7.2.5, 7.6.1). Because the
+// back with the generated password shown once (product.md, security.md). Because the
 // password cannot be recovered later, this deliberately renders inline rather
 // than redirecting.
 func (s *Server) handleAddApplication(w http.ResponseWriter, r *http.Request) {
@@ -253,7 +254,7 @@ func (s *Server) handleAddApplication(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
-// handleUpdateAppMode switches an application's address mode / list (spec 7.2.7).
+// handleUpdateAppMode switches an application's address mode / list (product.md).
 func (s *Server) handleUpdateAppMode(w http.ResponseWriter, r *http.Request) {
 	a, ok := s.lookupApplication(w, r)
 	if !ok {
@@ -282,7 +283,7 @@ func (s *Server) handleUpdateAppMode(w http.ResponseWriter, r *http.Request) {
 }
 
 // handleRegenPassword issues a new password for an application and shows it once
-// (spec 7.2.9, 7.6.1). Rendered inline, like creation, so the password is visible.
+// (product.md, security.md). Rendered inline, like creation, so the password is visible.
 func (s *Server) handleRegenPassword(w http.ResponseWriter, r *http.Request) {
 	a, ok := s.lookupApplication(w, r)
 	if !ok {
@@ -306,7 +307,7 @@ func (s *Server) handleRegenPassword(w http.ResponseWriter, r *http.Request) {
 }
 
 // handleDeleteApplication removes an application and returns to its domain page
-// (spec 7.2.8).
+// (product.md).
 func (s *Server) handleDeleteApplication(w http.ResponseWriter, r *http.Request) {
 	a, ok := s.lookupApplication(w, r)
 	if !ok {
@@ -343,7 +344,7 @@ func (s *Server) lookupApplication(w http.ResponseWriter, r *http.Request) (stor
 
 // splitAddresses turns the textarea/field input (addresses separated by
 // newlines, commas or whitespace) into a raw slice. Normalisation and
-// validation happen in the app service (spec 7.6.2).
+// validation happen in the app service (security.md).
 func splitAddresses(s string) []string {
 	return strings.FieldsFunc(s, func(r rune) bool {
 		return r == '\n' || r == '\r' || r == ',' || r == ' ' || r == '\t' || r == ';'