Mentorship: request lifecycle + capacity enforcement (Phase 6.3)

[pablo-clueless]

Scope

Mentee-initiated requests through to active mentorships.

Lifecycle

Pending → Accepted ⇒ ActiveMentorship | Declined(reason) | Withdrawn — then Active → Concluded (either side).

Data model

public sealed class MentorshipRequest
{
    public Guid Id { get; set; }
    public Guid MentorId { get; set; }
    public Guid MenteeId { get; set; }
    public string Message { get; set; }
    public RequestStatus Status { get; set; }     // Pending | Accepted | Declined | Withdrawn
    public string? DeclineReason { get; set; }
}

public sealed class Mentorship
{
    public Guid Id { get; set; }
    public Guid MentorId { get; set; }
    public Guid MenteeId { get; set; }
    public MentorshipStatus Status { get; set; }  // Active | Concluded
}

Endpoints (active-member policy)

Method	Path	Notes
POST	/mentorship/requests	mentee → mentor; message + goals
GET	/mentorship/requests/incoming / outgoing	paginated
POST	/mentorship/requests/{id}/accept	mentor only; creates Mentorship
POST	/mentorship/requests/{id}/decline	mentor only; reason optional but stored
POST	/mentorship/requests/{id}/withdraw	mentee only
GET	/mentorship/mine	active + concluded mentorships for caller
POST	/mentorship/{id}/conclude	either participant

Rules

• Capacity on accept: active mentorships < Capacity, else 409 mentorship.at_capacity. Hitting capacity auto-sets IsAcceptingMentees = false (manual re-enable when a mentorship concludes — do not auto-flip back).
• No duplicate open request to the same mentor; no self-mentorship; mentor must be accepting.
• Brevo (existing IEmailSender, best-effort): request received → mentor; accepted/declined → mentee.

Error codes

mentorship.not_found, mentorship.at_capacity, mentorship.not_accepting, mentorship.duplicate_request, mentorship.invalid_state, mentorship.forbidden

Dependencies

Blocked by #9.

Acceptance criteria

• Full transition matrix tested incl. wrong-party actions (mentee accepting, etc.)
• Capacity race: two concurrent accepts at capacity-1 → exactly one succeeds (constraint or transaction, tested)
• Emails logged-not-thrown on failure

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<T> (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/<Module>/ following the existing harness patterns (see Members/MembersTestHarness.cs).