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_status.go

@@ -4,7 +4,7 @@ import (
 	"net/http"
 	"strings"
 
-	"codeberg.org/mix/selfpost/internal/health"
+	"github.com/mixeme/selfpost/internal/health"
 )
 
 // handleStatus renders the server status page: the panel's landing page and the
@@ -26,7 +26,8 @@ func (s *Server) handleStatus(w http.ResponseWriter, r *http.Request) {
 }
 
 // handleStatusFragment serves the HTMX polling fragment for the local checks
-// (spec 7.1: fragment endpoints return HTML, not JSON).
+// (architecture.md § Panel HTTP surface: fragment endpoints return HTML, not
+// JSON).
 func (s *Server) handleStatusFragment(w http.ResponseWriter, _ *http.Request) {
 	s.renderFragment(w, http.StatusOK, "status_body", s.statusBody())
 }
@@ -90,9 +91,10 @@ func (s *Server) statusBody() map[string]any {
 	}
 }
 
-// queueSummary reduces postqueue's listing to the one line worth showing on the
-// status page; the full listing has its own screen (spec 7.2.11). postqueue
-// prints either "Mail queue is empty" or a trailing "-- N Kbytes in M Requests."
+// queueSummary reduces postqueue's listing to the one line worth showing on
+// the status page; the full listing has its own screen (architecture.md §
+// Panel HTTP surface). postqueue prints either "Mail queue is empty" or a
+// trailing "-- N Kbytes in M Requests."
 func queueSummary(out string) string {
 	lines := strings.Split(strings.TrimSpace(out), "\n")
 	for i := len(lines) - 1; i >= 0; i-- {