Carry the embargo invite RSVP deadline (Invite.end_time) through the wire layer

[sei-ahouseholder]

Summary

Carry the RSVP-by deadline through the wire layer for Invite(EmbargoEvent), per
CM-27-001, CM-27-006, EP-07-002, and EP-07-003.

Note that the embargo invitation is an as_Invite, not an as_Offer
(em_propose_embargo_activity() in vultron/wire/as2/factories/embargo.py
returns as_Invite). No vocabulary extension is needed: end_time is inherited
from as_Object (vultron/wire/as2/vocab/base/objects/base.py).

Critical hazard: an embargo invitation carries two end_time fields one
nesting level apart in the same JSON document — Invite.end_time (RSVP-by) and
Invite.object_.end_time (embargo expiry, on the nested as_EmbargoEvent).
These MUST be read independently and never substituted for one another.

Acceptance Criteria

• AC-1: em_propose_embargo_activity() accepts an optional RSVP deadline and
  sets it on the returned as_Invite's activity-level end_time (CM-27-001).
• AC-2: The extracted event for InviteToEmbargoOnCase exposes the
  activity-level end_time as a distinct field from the nested embargo's
  end_time. A test asserts both are readable and are not conflated.
• AC-3: An inbound Invite.end_time is normalised to UTC before storage or
  comparison; a naive (non-tz-aware) value is rejected as malformed rather than
  assumed UTC (CM-27-006, CS-13-001).
• AC-4: An outbound invite whose deadline is below the configured minimum
  window is refused at construction time (EP-07-002).
• AC-5: An inbound invite whose end_time is below the configured minimum is
  clamped up to the minimum, not rejected, and the clamp is logged
  (EP-07-003).
• AC-6: The minimum window and the fallback default window are configurable
  (AppConfig/ActorConfig, per CFG conventions), defaulting to 72h and 7 days
  respectively (EP-07-001, EP-07-002).
• AC-7: Tests cover absent end_time (fallback applies, current behaviour
  preserved), present-and-valid, sub-floor (clamped), and naive (rejected).

Reference

Source: #2066
Docs PR: #2210
ADR: docs/adr/0059-embargo-invite-rsvp-deadline.md
Specs: case-management.yaml CM-27-001, CM-27-006; embargo-policy.yaml EP-07
Notes: notes/participant-embargo-consent.md § "RSVP Deadlines on Embargo Invites"