Add Mailme adapter for email delivery

[joamag]

Problem (Why)

Pushi currently sends email only through raw SMTP, which requires each deployment to manage its own mail server credentials, deliverability and TLS configuration. Hive Solutions already runs Mailme, a simple REST email gateway with an official Python client (mailme-api), that removes that operational burden. Adding a Mailme adapter lets pushi apps send email through the managed gateway with just an API key, as an alternative to the SMTP handler.

Description (What)

Introduce a MailmeHandler adapter that delivers pushi events as emails via the Mailme REST gateway, with a Mailme subscription model, REST controller and API client mirroring the SMTP adapter. Recipient addresses are stored per subscription and overridable per published event, with subject/content derived from the event payload. Configuration uses Mailme's own convention (MAILME_BASE_URL, MAILME_KEY) alongside App-model fields, consistent with the existing credential pattern.

Implementation (How)

1. Add MailmeHandler(handler.Handler) in src/pushi/base/mailme.py implementing load, add, remove, subscribe, unsubscribe, send(app_id, event, json_d, invalid={}) and a direct send_to_receivers(...); honor per-event receiver/subject overrides over the stored subscription.
2. Prefer the mailme-api Python client (pip install mailme-api) if available, with a netius.clients.HTTPClient POST to MAILME_BASE_URL as a fallback; guard the import like the existing py_vapid optional dependency.
3. Add App-model config fields in src/pushi/app/models/app.py (mailme_base_url, mailme_key) with global env-var fallbacks (MAILME_BASE_URL, MAILME_KEY) via appier.conf.
4. Add the Mailme subscription model in src/pushi/app/models/mailme.py with fields (email, event) and handler-sync lifecycle hooks.
5. API: add a MailmeController in src/pushi/app/controllers/mailme.py exposing GET/POST /mailmes and DELETE /mailmes/<email>.
6. API client: add MailmeAPI in src/pushi/api/mailme.py with create_mailme / delete_mailme / subscribe_mailme / unsubscribe_mailme.
7. Register the handler in State.load_handlers() in src/pushi/base/state.py and export new symbols from the four __init__.py files.
8. Add mailme-api as an optional dependency in setup.py/requirements.txt.
9. Add tests in src/pushi/test/mailme.py (payload/auth, per-event override, subscription sync, optional-client fallback) using the mock-owner pattern.
10. Update README.md, add an examples/mailme/ example, and add a [Unreleased] CHANGELOG entry.