feat: initial ispconfig-py SDK (sites / dns / mail / databases / clients)

Python 3.10+ SDK wrapping the ISPConfig remote SOAP API so we stop writing
throwaway PHP snippets every time we need to touch a site, zone, or mailbox.

Why no zeep: ISPConfig's /remote/index.php exposes PHP SoapServer in non-WSDL
mode and refuses WSDL generation (?wsdl returns a fault). zeep requires WSDL,
so the stated dependency wouldn't actually work. Instead we hand-roll SOAP
envelopes with the stdlib (urllib + xml.etree). Zero runtime deps.

Structure:
- src/ispconfig/_soap.py        — envelope encode/decode, fault surfacing
- src/ispconfig/client.py       — ISPConfigClient context manager, retry
- src/ispconfig/exceptions.py   — ISPConfigError / Auth / Permission / NotFound / Fault
- src/ispconfig/sites.py        — web_domain get/add/update/delete + enable_php / enable_letsencrypt helpers
- src/ispconfig/dns.py          — zones + A/CNAME/MX/TXT records, dns_a_add type-column workaround
- src/ispconfig/mail.py         — mail domains, users, forwards, create_mailbox helper
- src/ispconfig/databases.py    — convenience facade over sites_database_*
- src/ispconfig/clients.py      — client + sys_groupid lookups
- src/ispconfig/types.py        — TypedDicts for response shapes (no pydantic)

Institutional knowledge baked into docstrings + README footgun list:
- sites_web_domain_update 2nd arg is client_id (not primary_id); admin = 0
- sys_groupid=1 -> pass client_id=0 on update, else ownership churns
- fastcgi_php_version vs server_php_id depending on panel version
- dns_a_add type-column bug (<= 3.2.11) — wrapper issues follow-up update
- dns_zone_get_id wants origin WITHOUT trailing dot on 3.2.11+ (contrary to
  what the older snippets say). Verified live against Rackham 2026-04-22.
- mail_user_get returns a bare map on exactly-one-match filter dicts —
  wrapper normalizes to list
- session timeouts mid-op: client detects + re-auths once (max_retries knob)

Tests:
- tests/test_unit.py   — 12 unit tests against a fake transport
- tests/test_smoke.py  — live read-only smoke test, gated on env vars:
    ISPCONFIG_TEST_URL, ISPCONFIG_TEST_USER, ISPCONFIG_TEST_PASS
  Covers login, web_domain_get(156), dns_zone_get_id, mail_user_get filter.

Tooling:
- mypy strict-ish (disallow untyped defs, warn-return-any, no implicit optional)
- ruff with E/F/W/I/B/UP/N/SLF/RUF lint sets
- pip install -e .[dev] for pytest / mypy / ruff

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Sulkta Coop
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.