Sponsorship: entities + Stripe catalog sync (Phase 7.1)

## Scope
New `CommunityPro.Sponsorship` module (schema `sponsorship`): tiers, sponsors, invoices, and the Stripe product/price catalog sync.

Tiers: **Community Supporter**, **Member Development Partner**, **Main Stage Sponsor** — recurring subscriptions with monthly + annual prices, plus a one-off "custom contribution" path. *(Flagged assumption from the spec: if tiers turn out to be one-off, Checkout mode changes in the follow-up ticket — the model here is unaffected.)*

## Data model
```csharp
public sealed class SponsorshipTier
{
public Guid Id { get; set; }
public string Name { get; set; }
public string[] Benefits { get; set; } // copy lives in DB; prices live in Stripe
public string StripeProductId { get; set; }
public string StripeMonthlyPriceId { get; set; }
public string StripeAnnualPriceId { get; set; }
}

public sealed class Sponsor
{
public Guid Id { get; set; }
public string OrgName { get; set; }
public string ContactEmail { get; set; }
public string StripeCustomerId { get; set; }
public string? StripeSubscriptionId { get; set; }
public Guid TierId { get; set; }
public SponsorStatus Status { get; set; } // Active | PastDue | Cancelled
public string? LogoUrl { get; set; } // Cloudinary
public bool ShowInGallery { get; set; }
}

public sealed class SponsorInvoice
{
public Guid Id { get; set; }
public Guid SponsorId { get; set; }
public string StripeInvoiceId { get; set; }
public long AmountCents { get; set; }
public string Currency { get; set; }
public InvoiceStatus Status { get; set; }
public string HostedInvoiceUrl { get; set; }
public string PdfUrl { get; set; }
public DateTimeOffset IssuedAt { get; set; }
}
```

## Catalog sync
`dotnet run -- sync-stripe-catalog`: idempotently ensures Stripe Products + Prices exist for each tier from config, storing the resulting ids in the DB. **Environment-aware — never hardcode price ids** (test vs live keys produce different ids). Stripe SDK calls behind a module-owned interface.

## Acceptance criteria
- [ ] Sync is idempotent (re-run → no duplicate products/prices)
- [ ] Tier seed data (names + benefits) migrated
- [ ] `env.example` gains Stripe keys
- [ ] xUnit coverage with a fake Stripe client

### Conventions (project-wide, non-negotiable)
- .NET 9, records for immutable shapes, file-scoped namespaces, primary constructors where they read well. Minimal-API endpoints grouped per module via `IEndpointModule.MapEndpoints`.
- `Result` (SharedKernel) instead of exception-driven control flow. Endpoint results map failures to ProblemDetails with the stable error codes listed above — the frontend keys off them.
- Module owns its EF Core `DbContext` mapped to its own Postgres schema. Modules never reference each other's internals — cross-module needs go through a public contract interface or an in-process domain event (`IEventPublisher`).
- All external calls (GitHub, Stripe, Brevo, Cloudinary, Meilisearch) behind interfaces owned by the consuming module.
- Every list endpoint paginated (offset is fine). xUnit tests in `tests/CommunityPro.Tests//` following the existing harness patterns (see `Members/MembersTestHarness.cs`).