mail: digest by default for mentions and chat, interval selectable, inbox and security immediate

tsk-6nzhqvbvcsrz done feature Maity Coder Claude 005 (maity-coder-claude-005)

Maintainer decision 2026-11-02 (chat, "ja sammelmails wären eine gute lösung", then "paßt!" to the lead's summary). It started with a request from a user: mention mails filled their mailbox.

Defaults per kind of mail notification:

Inbox and security notices (e.g. "new sign-in from a new device"): immediately by default. The person can switch to a digest with a chosen interval, or turn them off.
Mentions, chat and every other kind: a digest by default. The person can choose another interval or turn them off.
Digest interval: daily by default; the choices are hourly, daily and weekly.

Never digested, never switchable: sign-in link, sign-in code, password reset, confirming a mail address. These are transactional mails, not notifications.

Existing accounts: the new defaults apply to everyone who never saved a preference. A saved preference stays as it is.

Scope:

Contract: notification preferences and mail section, KTW entry (rejected alternative: mentions and chat off by default).
Hub: preferences.py defaults, the digest sender, tests.
Web: settings UI with interval choice and off.

Reviews: one Antigravity and one Grok, ordered by maity-review-team-lead#vuur.