feat(web): one notification feed of every daemon's notices

[futrime]

Unit 5 of the multi-daemon + notifications plan: contract C7 on the local server.

What

• New src/hmz/web/notifications.py:
  • GET /api/notifications returns {items, unread}, newest first. Each item is a C4 notice plus its own id, daemon (local or the entry name) and read. The last 200 are kept in memory.
  • GET /api/notifications/stream sends SSE notification events and resumes by Last-Event-ID or ?last=. A page whose last id came from an earlier server starts over.
  • POST /api/notifications/read takes {ids} or {all: true}. POST /api/notifications/clear empties the feed.
  • GET and POST /api/notifications/settings read and write C2 under notifications in settings.yaml, through coganchor.settings.read and .changes. All kinds default to true. Anything else gets a 400 and nothing is written.
• Sources:
  • Local site.notices (C4 .heard).
  • Each SSH daemon named in settings.yaml daemons (C1) whose site.remotes.state(name) is up. It is followed through reached(name): sign in with /?key= the way a browser does, which yields the remote's own hmz-<port> cookie. Then read /api/notices for last, then follow /api/notices/stream.
  • Both sources are looked up with getattr every 2 s, so this is green on its own and picks up the sibling units once they land.
• Kinds are filtered by C2 before an item is stored. Preferences are re-read for each notice, so a change applies at once.
• Docs: docs/reference/web.md (Notifications API) and docs/reference/settings.md (notifications key).

Tests

• tests/unit/web/test_notifications.py: public names, with fake notices.
• tests/integration/test_web_notifications.py: the served site with a fake site.notices, a fake site.remotes, and a stand-in remote interface on a local port. Covers the cross-daemon feed, filtering, settings written to settings.yaml, refusals, read, clear, and the SSE stream with resume.
• Live: hmz web --port 0, then curl every endpoint and check settings.yaml.

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[futrime]

Landed together in #385.