relay: enforce ban/timeout write-block on command kinds

[Maxwellimus]

Summary

ingest_event routes command kinds — DM open/add/hide, workflow
define/trigger, and approval grant/deny — straight to
command_executor::handle_command and returns before the durable
ban/timeout write-block gate that ordinary writes pass through.
handle_command performs no restriction check of its own, so a member
who has been banned or timed out could still:

• open, add to, or hide DM channels,
• define and trigger workflows,
• grant or deny approvals,

by sending the corresponding signed event over POST /events (or the
WebSocket). The moderation-command and relay-admin kinds are also routed
around the shared gate, but each re-checks the ban inside its own
handler; command kinds were the one exempt path with no compensating
enforcement.

Fix

Enforce the durable ban/timeout write-block on command kinds before
routing to the executor:

• A banned member's command kind is refused 403 blocked:.
• A timed-out member's command kind is refused 403 restricted:.
  Command kinds are ordinary writes, not administrative capability, so
  an active timeout blocks them — the relay-admin timeout exemption does
  not extend here.
• An unrestricted member is unaffected.

The ban/timeout decision is extracted into restriction_block /
enforce_moderation_restriction, now the single source of truth shared
by both the command-kind path and the ordinary-write path, so the two
can't drift. The restriction-state lookup fails closed: a DB error
surfaces as 500 error: rather than admitting the write.

Tests

• Unit tests for restriction_block: banned → blocked:, active
  timeout → restricted:, expired timeout → no block, unrestricted → no
  block.
• New #[ignore]d e2e regression test
  (regression_command_kind_ban_gate.rs): against a live relay, a
  banned member and a timed-out member each have their DM_OPEN command
  refused (403 with the exact wire contract) and create no DM channel,
  while an unrestricted member's DM_OPEN still succeeds and mutates.
  Pre-fix, the banned member's DM_OPEN is accepted (200) — the test
  fails on that assertion.

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