Parallelise per-phase BoxProvisioningHostedService for multi-tenant deployments

[iancooper]

Context

PR #4039's BoxProvisioningHostedService.StartAsync provisions all
registered boxes sequentially in a single foreach:

// src/Paramore.Brighter.BoxProvisioning/BoxProvisioningHostedService.cs:82-90
var ordered = _provisioners.OrderBy(p => p.BoxType); // Outbox before Inbox
foreach (var provisioner in ordered)
{
    await provisioner.ProvisionAsync(cancellationToken);
}

For a single-database deployment this is correct (the database-level
migration lock would serialise concurrent provisioners anyway).
Multi-tenant deployments with N independent databases pay a linear
startup cost: total time = Σ per-tenant migration time, where
ideally it should be max(per-tenant migration time).

What needs doing

Group provisioners by BoxType and run each phase with
Task.WhenAll, preserving the Outbox-before-Inbox ordering required
by the API contract:

var byPhase = _provisioners.GroupBy(p => p.BoxType).OrderBy(g => g.Key);
foreach (var phase in byPhase)
{
    await Task.WhenAll(phase.Select(p => p.ProvisionAsync(cancellationToken)));
}

Considerations:

• Per-tenant migration locks remain per-database, so parallelism does
  not affect lock semantics.
• Exception aggregation: Task.WhenAll aggregates faulted tasks into
  a single AggregateException. The current sequential code fails
  fast on the first throw. Decide whether to preserve fail-fast (use
  Task.WhenEach or a custom wait pattern) or accept aggregation.
• Cancellation propagation: each ProvisionAsync already accepts a
  shared CancellationToken; no change needed.
• Logging shape: the existing per-provisioner Information logs will
  interleave; consider whether to keep that or wrap each Task with a
  scoped logger.

Why this is a follow-up, not part of PR #4039

The reviewer flagged this as observational ("Worth deferring unless
multi-tenant scenarios surface as a real concern"). No deployment
in-scope for PR #4039 exercises the multi-DB path.

Acceptance

• Per-phase parallel provisioning behind Task.WhenAll.
• Tests covering: (a) parallel exec preserves Outbox-before-Inbox
  ordering, (b) exception in one provisioner does not prevent siblings
  from finishing (or, if fail-fast preferred, that the first failure
  cancels siblings).
• No regression on single-database deployments.

Reference

PR #4039 review comment:
#4039 (comment)

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