release: 2.0.0: Panel redesign; new URLs; database from zero

The panel redesign (docs/plans/panel-redesign.md, 27/27). A breaking
release: every panel URL changed and the database starts from a single
baseline - a 1.x /data is not upgraded.

Since the last cut also fixed, all found on the test server:
  - the panel can read the mail queue;
  - port 25 answers with DMARC report ingest on, also next to the
    inbound relay;
  - DMARC reports reach the panel (the delivery service never worked
    since 1.7.0);
  - Postfix start-up warnings about the queue directory.

Cut: CHANGELOG [Unreleased] -> [2.0.0]; image pin 2.0.0 in compose,
README, guide; roadmap, plan and version table.

On the test server, on 252e5d1: go vet ./... ok, go test ./... ok
(22 packages), make e2e ok.

Full detail: CHANGELOG.md (2.0.0).

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 `1.9.5`. Intermediate
+deliberately never `:latest`. The current pin is `2.0.0`. 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