Keep sign-ups unlinkable on ActivityPods, and land the pending coherence-pass work

Participation privacy (decision_2026-10-29_participation-privacy-mechanism):
a sign-up is a private Join naming the meeting point, addressed to nobody,
handled by the backend on emission. Attendee lists hold member URNs hashed
from (WebID, event, a secret kept in the participant's profile), so counts
stay public and only contacts recognise anyone; the creator has no read
right and is notified anonymously. Each person's participations collection,
created by the backend at the first sign-up, is readable by their /contacts
WAC group only, never world-readable. The member URN is sticky per
(meeting point, participant) and every collection write is verified by
reading back, so a refused write leaves the act unrecorded and retryable.
The EventRef mechanism and its browser-side provisioning are gone.

The backend now runs with WebACL on, as the reference apps do, on its own
secure datasets festipod / settings-festipod instead of the pod provider's
settings dataset; pod-notifications needs it. The compose header and the
deployment leaf say how to reset a stack whose datasets predate this.

The rights matrix covers the new mechanism live (148 hard lines), including
a forged contact Accept between throwaway accounts: the stranger enters the
contacts list (platform defect, reported upstream) but is refused the profile
and the participations. Tests may attack as long as they never break the dev
environment (rule_tests-never-break-the-dev-environment).

Also lands the coherence-pass work that was still uncommitted: contacts as
the Pod's social graph and invite links, image upload to the Pod, the
meeting-point form, profile completion gate, and the doctrine for them.
Superseded briefs and decisions are deleted rather than annotated.

backend/services/keyed-serializer.js

@@ -35,8 +35,10 @@
 
 // 30 s: a serialized decision makes a handful of pod round-trips — read the object, read up to two
 // ACLs, write up to two (public-read); read, then one store write (catalog); a wait of at most 1.5 s
-// for a fresh collection, a read and one write (attendance) — each answering in tens of milliseconds
-// on the local stack, a few seconds at worst on a loaded one. 30 s is over ten times the slowest
+// for a fresh collection, the participant's participations collection made ready (a profile and an
+// ACL read; at its creation, the create-and-attach, a profile re-read, one ACL write and one group
+// grant), a read, two writes and their two read-backs (attendance) —
+// each answering in tens of milliseconds on the local stack, a few seconds at worst on a loaded one. 30 s is over ten times the slowest
 // legitimate run, and short enough that a stalled pod holds one object's decisions for half a minute
 // rather than until the backend restarts.
 const DEFAULT_TIMEOUT_MS = 30000;