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>

README.md

@@ -112,7 +112,7 @@ docker run --rm -d --name selfpost-try \
   -e SELFPOST_HOSTNAME=mail.local.test \
   -e PANEL_COOKIE_SECURE=false \
   -v selfpost-try-data:/data \
-  ghcr.io/mixeme/selfpost:1.9.5
+  ghcr.io/mixeme/selfpost:2.0.0
 ```
 
 **Get the setup URL** (pick one):