feat(mentor): bulk assignment + coalesced emails + team intros on round open

Round-page bulk-assign UI
- Checkboxes on every project row, header select-all, primary-tinted action
  toolbar that appears when 1+ rows are selected with an "Assign mentor…"
  CTA and Clear. Dialog lists the mentor pool with search (name/email/
  country/expertise), load indicator, and a radio picker.
- Always-visible tip strip when nothing is selected explains the bulk flow
  and offers a one-click "Select all N without a mentor" shortcut.
- New tRPC procedure `mentor.bulkAssign({ mentorId, projectIds })` assigns
  one mentor to many projects in a transaction; idempotent on the per-pair
  `(projectId, mentorId)` unique; per-project in-app notifications still
  fire for each team.
- Mutation invalidates listMentoringProjects, getProjectsNeedingMentor,
  getMentoringImportCandidates, getMentorPool, getRoundStats, project.list
  so the page reflects the new state without a refresh.

Coalesced mentor emails
- New `sendMentorBulkAssignmentEmail` (single email listing every newly-
  assigned project + workspace links) used by `mentor.bulkAssign` and
  `mentor.autoAssignBulkForRound`. The previously-silent auto-fill flow
  now emails mentors at the end of the batch, one combined email per
  mentor regardless of how many projects they received.

Team introduction emails when the round opens
- New `sendTeamMentorIntroductionEmail` lists every assigned mentor with
  name + email and a link to the workspace, so teams can reach out
  directly.
- `activateRound` (round-engine) fires the introduction for every project
  in a MENTORING round that has active mentors when the round opens.
- `mentor.assign`, `mentor.bulkAssign`, and `autoAssignBulkForRound` also
  fire the introduction immediately when the project's MENTORING round is
  already ROUND_ACTIVE — so mentors added mid-round still reach the team.
- Idempotency via the new `MentorAssignment.teamIntroducedAt` column
  (migration 20260526114936) — independent from `notificationSentAt` so
  pre-existing mentor-side stamps don't suppress the team-side email.

prisma/schema.prisma

@@ -1281,9 +1281,16 @@ model MentorAssignment {
   assignedAt DateTime               @default(now())
   assignedBy String? // Admin who assigned
 
-  // Per-assignment email idempotency: stamped once the assignment notification email is sent.
+  // Per-assignment email idempotency: stamped once the MENTOR-side notification
+  // email has been sent (the "you've been assigned a project" email to the mentor).
   notificationSentAt DateTime?
 
+  // Stamped once the TEAM has been introduced to this mentor (the "meet your
+  // mentor" email with mentor contact info). Fired by `activateRound` for
+  // MENTORING rounds and by mentor.assign when the project's MENTORING round
+  // is already ROUND_ACTIVE. Independent from notificationSentAt above.
+  teamIntroducedAt DateTime?
+
   // AI assignment metadata
   aiConfidenceScore   Float?
   expertiseMatchScore Float?