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>

docs/guide.md

@@ -183,7 +183,7 @@ cat ./data/setup-token
 #### Fixed image tag
 
 `deploy/docker-compose.yml` pins an explicit version (`ghcr.io/mixeme/selfpost:X.Y.Z`),
-deliberately never `:latest`. The current pin is `2.0.0`. Intermediate
+deliberately never `:latest`. The current pin is `2.0.1`. Intermediate
 CHANGELOG sections (`0.2.0`…`0.6.0`) record development cuts from before that
 image was published. Pinning matters because of the backup version check (see
 [Full backup and restore](#full-backup-and-restore)): the panel binary's