fix(hosts): rate-limit heartbeats and close enrollment and audit gaps

Three control-plane gaps:

Heartbeats have no minimum interval (apps/control_plane/lib/workharbor/host_connections.ex, around lines 215-247). Each one writes a transaction plus an undeletable audit row, so a buggy or compromised agent can grow the audit table without bound and starve owner writes (persistence_busy).
Issuing a new enrollment code for a host leaves earlier unexpired codes valid (hosts.ex, around lines 15-52). Redeeming the stale code retires the identity the agent just enrolled with.
authorize! inside a transaction calls Repo.rollback(:forbidden), which also rolls back the authorization_denied audit (hosts.ex around 18/242/374, config_trust.ex around 29/111). Denial audits are security evidence under authentication.md §4.

Acceptance criteria

Heartbeats faster than a fixed floor (for example 5 s against the 10 s cadence) are refused or close the connection, with a test.
Issuing a code revokes the host's outstanding unconsumed codes, with a test.
A denial inside a transaction keeps its audit row, with a test.
make check is green.

Found by the independent pilot review on 2026-10-16. Builds on the pilot branch pilot/installable-pilot-pr (split from #83); start from its current head.

🤖 Generated with Claude Code

fix(hosts): rate-limit heartbeats with a receive-time token bucket