feat: add async-daemon opt-in to AppFoundationOptions

[andregoepel]

Summary

AddAppFoundation()'s single AddMarten(...) call never called AddAsyncDaemon(...) and exposed no hook to add one — customer-portal ships its own MartenAsyncDaemonHostedService workaround as a result (tracked separately: customer-portal#90).

Changes

• AppFoundationOptions.EnableAsyncDaemon (bool, default false — no behavior change for existing consumers) and AppFoundationOptions.AsyncDaemonMode (DaemonMode, default Solo).
• Initialization.cs captures AddMarten(...)'s return value and conditionally calls .AddAsyncDaemon(options.AsyncDaemonMode) before .IntegrateWithWolverine().

Design note

AddMarten(...).AddAsyncDaemon(mode) is Marten's own registration point — confirmed via reflection that it registers ProjectionCoordinator as an IHostedService, so enabling the option is sufficient on its own; no extra hosted service needed on our side. Solo is the sensible default for a single running instance; HotCold is available for hosts running multiple instances that want leader election.

Verification

• dotnet csharpier format . clean
• dotnet build — 0 warnings, 0 errors
• dotnet test — 89 tests passing (4 new), covering default-disabled / enable-defaults-to-Solo / explicit-mode-override / mode-without-enabling-has-no-effect
• dotnet list package --vulnerable --include-transitive — clean

Closes #153