Admin: mentorship + sponsorship oversight endpoints (Phase 8.4, 8.5)

## Scope
Read-heavy admin surfaces for the dashboard, plus one intervention action.

## Mentorship oversight
| Method | Path | Notes |
|---|---|---|
| GET | `/admin/mentorship/pairs` | active pairs, paginated |
| GET | `/admin/mentorship/sessions?upcoming=true` | upcoming sessions |
| GET | `/admin/mentorship/mentors` | capacity overview (capacity vs active count) |
| POST | `/admin/mentorship/{id}/conclude` | conclude-on-behalf for disputes (audit-logged via Serilog structured fields) |

## Sponsorship oversight
| Method | Path | Notes |
|---|---|---|
| GET | `/admin/sponsors` | list with status, tier, subscription state |
| GET | `/admin/sponsors/summary` | **MRR computed from active subscriptions** (annual normalized to monthly) + counts by status |
| GET | `/admin/sponsors/{id}/invoices` | mirrored invoices with status + Stripe hosted links |

## Rules
- Endpoints live in the **owning modules** under `/admin/*` with the admin policy at group level (#1's pattern) — the Admin module itself owns nothing here.
- All list endpoints paginated; summaries are single cheap queries.

## Error codes
`admin.forbidden`, `mentorship.not_found`, `sponsorship.not_found`

## Dependencies
Blocked by #1; mentorship endpoints need #10/#11; sponsorship endpoints need #16.

## Acceptance criteria
- [ ] MRR math tested: monthly + annual mix, PastDue excluded
- [ ] Conclude-on-behalf emits the same events/side-effects as participant conclusion
- [ ] Non-admin blocked on every endpoint

### 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`).