release: 2.0.1: DMARC ingest drops and bounds; Full backup from the panel

A patch on 2.0.0, all of it found on the test server after the cut:
  - a message that is not a report this relay keeps is dropped at once
    instead of being retried for days;
  - a report is bounded before it is stored, and reports are kept per
    domain (security review of the ingest path, nothing Critical or High);
  - Full backup from the panel works (it had answered 200 with ten bytes
    since 1.3.0);
  - no backup holds the Postfix queue, and the panel says why.

Cut: CHANGELOG [Unreleased] -> [2.0.1]; image pin 2.0.1 in compose,
README, guide; version table.

On the test server, on dd0fefd: go vet ./... ok, go test ./... ok
(22 packages), make e2e ok.

Full detail: CHANGELOG.md (2.0.1).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/docker-compose.yml

@@ -21,7 +21,7 @@
 
 services:
   selfpost:
-    image: ghcr.io/mixeme/selfpost:2.0.0
+    image: ghcr.io/mixeme/selfpost:2.0.1
     restart: unless-stopped
     environment:
       SELFPOST_HOSTNAME: "${SELFPOST_HOSTNAME:?set the mail/panel hostname, e.g. mail.example.com}"