Mentorship: session booking + ICS emails (Phase 6.4)

[pablo-clueless]

Scope

Book sessions against mentor availability, with calendar-attachment emails.

Data model

public sealed class Session
{
    public Guid Id { get; set; }
    public Guid MentorshipId { get; set; }
    public DateTimeOffset StartUtc { get; set; }  // stored UTC, always
    public int DurationMinutes { get; set; }      // 30 | 60
    public SessionStatus Status { get; set; }     // Scheduled | Completed | Cancelled
    public string? MeetingUrl { get; set; }       // mentor-provided free text (Meet/Zoom)
}

Endpoints (participants of the mentorship only)

Method	Path	Notes
GET	/mentorship/{id}/slots?weekOf=	open slots: availability minus existing sessions, computed in the mentor's IANA tz via NodaTime, returned as UTC instants
POST	/mentorship/{id}/sessions	book; validations below
POST	/mentorship/sessions/{id}/cancel	either participant, only while Scheduled

Booking validations

1. Slot falls entirely inside the mentor's availability (evaluated in mentor-local time — DST-correct via NodaTime).
2. No overlap with any of the mentor's existing Scheduled sessions (across all their mentorships).
3. Start ≥ 12h in the future.
4. Duration ∈ {30, 60}; mentorship is Active.

Emails

Booked/cancelled → both parties via Brevo, with an .ics attachment generated via Ical.Net (UTC times, summary, meeting URL when present). Best-effort as always.

Error codes

mentorship.not_found, mentorship.slot_unavailable, mentorship.slot_conflict, mentorship.too_soon, mentorship.invalid_state, mentorship.forbidden

Dependencies

Blocked by #9 and #10.

Acceptance criteria

• DST boundary tests are mandatory: a slot spanning a DST transition in e.g. Europe/London or America/New_York books correctly; test both spring-forward and fall-back weeks
• Overlap check covers sessions from the mentor's other mentorships
• Concurrent booking of the same slot → exactly one succeeds
• ICS content asserted in tests (times, attendees)

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).