feat: support a default expiry for organisation invite links

[cloorc]

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Changes

InviteLink.expires_at is nullable and, when left blank, the link never
expires (is_expired returns False while expires_at is None). There is
currently no way for an operator to make invite links expire by default — each
link would have to be created with an explicit expires_at.

This PR adds an opt-in INVITE_LINK_EXPIRY_DAYS setting:

• When set, a newly created InviteLink without an explicit expires_at
  defaults to now + INVITE_LINK_EXPIRY_DAYS days (applied via a
  BEFORE_CREATE lifecycle hook).
• The setting defaults to None, preserving today's indefinite behaviour, so
  this is fully backward compatible.
• An explicitly provided expires_at is never overridden.

How did you test this code?

Added unit tests in test_unit_invites_models.py:

• …default_expiry_setting__sets_expires_at — with the setting configured, a
  link created without an expiry gets expires_at ≈ now + N days.
• …no_default_expiry_setting__stays_indefinite — with the setting unset, a
  link created without an expiry stays expires_at = None.
• …explicit_expiry__not_overridden_by_default — an explicit expires_at is
  preserved even when the setting is configured.

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