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/templates.go

@@ -9,21 +9,23 @@ import (
 
 // templates holds the parsed page and fragment templates. Each page is parsed
 // together with the shared base layout so {{ template "base" . }} works.
-// Fragments (HTMX polling targets, spec 7.1) are parsed standalone, without
-// the layout, so they can be swapped into an existing page as an HTML snippet
-// rather than a full document. Rendering always goes through html/template,
-// which auto-escapes all interpolated data regardless (spec 7.6.7).
+// Fragments (HTMX polling targets, architecture.md § Panel HTTP surface) are
+// parsed standalone, without the layout, so they can be swapped into an
+// existing page as an HTML snippet rather than a full document. Rendering
+// always goes through html/template, which auto-escapes all interpolated data
+// regardless (security.md).
 type templates struct {
 	pages     map[string]*template.Template
 	fragments map[string]*template.Template
 }
 
 // pageFiles maps a logical page name to its template files. Every page
-// composes with layout.html; pages that embed a polling fragment (spec 7.1)
-// list that fragment's file too, so the same {{define}} block renders both
-// the initial page and the fragment's own refresh responses identically. Pages
-// sharing a block of markup (the encryption fields on the two secret downloads)
-// list that partial the same way.
+// composes with layout.html; pages that embed a polling fragment
+// (architecture.md § Panel HTTP surface) list that fragment's file too, so the
+// same {{define}} block renders both the initial page and the fragment's own
+// refresh responses identically. Pages sharing a block of markup (the
+// encryption fields on the two secret downloads) list that partial the same
+// way.
 var pageFiles = map[string][]string{
 	"setup":         {"templates/setup.html"},
 	"login":         {"templates/login.html"},
@@ -101,7 +103,8 @@ func (s *Server) render(w http.ResponseWriter, status int, page string, data any
 }
 
 // renderFragment writes an HTMX polling fragment as a bare HTML snippet, with
-// no surrounding layout (spec 7.1: fragment endpoints return HTML, not JSON).
+// no surrounding layout (architecture.md § Panel HTTP surface: fragment
+// endpoints return HTML, not JSON).
 func (s *Server) renderFragment(w http.ResponseWriter, status int, name string, data any) {
 	tmpl, ok := s.tmpl.fragments[name]
 	if !ok {