feat: add AddStandardMailHog/AddStandardPostgres Aspire AppHost helpers

[andregoepel]

Summary

5 AppHost.cs files across the ecosystem (andregoepel-dev, finance-app, customer-portal, this repo's own sample, marten-identity's sample) hand-roll ~65%-identical MailHog + Postgres + EmailSender__* blocks. New packable project AndreGoepel.AppFoundation.Aspire extracts the container-wiring core.

Two things this fixes along the way

MailHog endpoint name standardized on "http" (not "web") — matches AndreGoepel.Testing.E2E's E2EAppFixtureOptions.MailHogEndpointName default, shipped in Wave 1. andregoepel-dev/finance-app currently use "web"; customer-portal/this repo already used "http". Adopting this helper fixes the inconsistency as a side effect.

isE2E gate exists everywhere now. AddStandardPostgres(isE2E, ...) skips WithDataVolume, persistent lifetime, and the fixed host port when isE2E is true. Confirmed by reading all 5 AppHosts: finance-app already gates all three; marten-identity gates the volume; andregoepel-dev and customer-portal currently have no isE2E gate at all — their Postgres always persists regardless of E2E mode. That gap is exactly what customer-portal#91 and andregoepel-dev#58 (both already filed) should pick up when they adopt this package.

Design

• AddStandardMailHog(builder, name, tag, smtpPort, httpPort) returns the container resource so callers can still wire up their own app-specific EmailSender__* env vars (sender name/credentials stay app-specific, not standardized).
• AddStandardPostgres(builder, isE2E, ...) returns a StandardPostgres(Server, Database) record, deconstructible so the common case stays a one-liner: var (_, db) = builder.AddStandardPostgres(isE2E);.
• Lives in this repo as a new package (not app-foundation's runtime packages) since AppHost projects are dev-only orchestration outside the core → marten-* → app-foundation → host apps dependency order the root CLAUDE.md governs — same reasoning that already puts ServiceDefaults here.
• This repo's own sample AppHost.cs migrated to dogfood both helpers.

Verification

• dotnet csharpier format . clean
• dotnet build — 0 warnings, 0 errors
• dotnet test — 119 tests passing total (5 new, resource-model only — no Docker needed): MailHog endpoint names/ports, Postgres isE2E gating of volume/lifetime, database-name-vs-resource-name distinction
• dotnet list package --vulnerable --include-transitive — clean

Closes #154. Follow-ups tracked separately: finance-app#69, customer-portal#91, andregoepel-dev#58 (isE2E gate adoption + endpoint rename in the 3 repos still on the old pattern).