Mentorship: messaging thread + unread tracking (Phase 6.5)

[pablo-clueless]

Scope

Per-mentorship message thread. Polling v1 — no websockets (frontend polls with refetchInterval: 5000 while the thread is open).

Data model

public sealed class ThreadMessage
{
    public Guid Id { get; set; }
    public Guid MentorshipId { get; set; }
    public Guid SenderId { get; set; }
    public string Body { get; set; }
    public DateTimeOffset SentAt { get; set; }
}
// plus LastReadAt per participant (e.g. MentorshipParticipantState) for unread counts

Endpoints (participants only)

Method	Path	Notes
GET	/mentorship/{id}/messages?after=	incremental fetch for polling; after = cursor (message id or timestamp); capped page size
POST	/mentorship/{id}/messages	body length-capped (~2000 chars), trimmed, non-empty
POST	/mentorship/{id}/read	stamp caller's LastReadAt

Unread counts (per mentorship, for the dashboard) derived from LastReadAt — expose on the GET /mentorship/mine payload from #10 rather than a separate endpoint.

Rules

• Only Active mentorship participants can post; reading history stays allowed after conclusion.
• Plain text only in v1 — no HTML rendering path at all.

Error codes

mentorship.not_found, mentorship.forbidden, mentorship.invalid_message, mentorship.invalid_state

Dependencies

Blocked by #10.

Acceptance criteria

• after cursor returns strictly newer messages, stable ordering
• Non-participant gets not_found-shaped forbidden (no existence leak)
• Unread counts correct across both participants
• Posting to a concluded mentorship rejected

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).