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/milter/ratelimit.go

@@ -4,19 +4,20 @@ import (
 	"log"
 	"time"
 
-	"codeberg.org/mix/selfpost/internal/store"
+	"github.com/mixeme/selfpost/internal/store"
 )
 
 // overLimit reports whether the message currently being received should be
-// refused under a level-2 differentiated limit (spec 7.4). It checks the
-// domain-level and application-level limits in turn; either being exceeded is
-// enough to refuse.
+// refused under a level-2 differentiated limit (README § Rate limiting). It
+// checks the domain-level and application-level limits in turn; either being
+// exceeded is enough to refuse.
 //
 // It is deliberately fail-open: any store error, or the absence of a usable
-// limit, is treated as "not over limit" so a malfunction of the level-2 limiter
-// can never block mail — Postfix's level-1 anvil limit (spec 5) remains the
-// backstop, and it does not depend on this milter at all. Only a clean count at
-// or above a configured ceiling returns true.
+// limit, is treated as "not over limit" so a malfunction of the level-2
+// limiter can never block mail — Postfix's level-1 anvil limit
+// (architecture.md § Mail path) remains the backstop, and it does not depend
+// on this milter at all. Only a clean count at or above a configured ceiling
+// returns true.
 //
 // A message that passes reserves a slot per applicable limit, released once it
 // reaches the send log (or is abandoned) — see inflight for why the stored
@@ -78,10 +79,10 @@ func (s *session) releaseReservations() {
 	s.reserved = nil
 }
 
-// recordRejected writes a send-log row for a message refused by a level-2 limit
-// (spec 7.4, "опционально фиксирует ... для видимости в UI"), so the rejection
-// shows up in the monitoring screen. Only MAIL-stage fields are known; the write
-// is best-effort and never affects the response.
+// recordRejected writes a send-log row for a message refused by a level-2
+// limit (README § Rate limiting — refusals are recorded too), so the rejection
+// shows up in the monitoring screen. Only MAIL-stage fields are known; the
+// write is best-effort and never affects the response.
 func (s *session) recordRejected() {
 	err := s.rec.InsertRejected(store.SendLogEntry{
 		Domain:   domainOf(s.from),