refactor: extract DirectAdminClient into directdnsonly.app.da module 🏗️

Move all outbound DirectAdmin HTTP logic out of ReconciliationWorker and
into a dedicated, independently testable DirectAdminClient class:

- directdnsonly/app/da/client.py: list_domains (paginated JSON + legacy
  fallback), get (authenticated GET to any CMD_* endpoint), _login
  (DA Evo session-cookie fallback), _parse_legacy_domain_list
- directdnsonly/app/da/__init__.py: public re-export of DirectAdminClient
- reconciler.py: now purely reconciliation logic; instantiates a client
  per configured server — no HTTP code remaining
- tests/test_da_client.py: 16 dedicated tests for DirectAdminClient
- tests/test_reconciler.py: mocks at the DirectAdminClient class boundary
  instead of the internal _fetch_da_domains method

Bumped to 2.2.0 — DirectAdminClient is now a first-class public API.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "directdnsonly"
-version = "2.1.0"
+version = "2.2.0"
 description = "DNS Management System - DirectAdmin to multiple backends"
 authors = [
     {name = "Aaron Guise",email = "aaron@guise.net.nz"}