fix(hub): an administrator can erase an account that owns groups

An administrator's deletion answered 409 for any account owning a group,
so an erasure ordered by an authority had to wait on the person it was
about. It now deletes the account's groups with it, then pushes a signed
revocation for the account and for each group to every connected node:
an access token already issued stays valid on a node until it expires,
and the revocation is what makes the nodes refuse the account and close
the groups' sessions now. The action is written to the IP log, and the
confirmation dialog says the groups go too, in all ten catalogues.

The owner's own deletion is unchanged: refused while they own groups,
which they can hand over first (CGU 3.4, privacy statement).

Deleting a group had three partial cascades. The owner's route left
email_verifications behind, and the cleanup of unhosted groups left
notifications, invitations and reports - each an IntegrityError on
PostgreSQL, invisible on SQLite, which does not enforce foreign keys by
default. db/purge.py is now the one implementation: it finds every table
referencing groups.id from the schema, deletes the group's rows and
detaches content reports, which are evidence and outlive the group.

test_group_purge.py turns foreign-key enforcement on for its connection,
seeds every referencing table, and fails without the fix on all three
routes. MESHBAY_DESIGN.md 7.7 states the rule, and now lists the device
keys and swarm sources that e3c68b3 erases.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D9MCBBWSm9GhBESmqzJxNy

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