feat(schema): multi-mentor per team + change-requests + per-assignment email field

- MentorAssignment: drop projectId @unique -> composite (projectId, mentorId)
- MentorAssignment: add notificationSentAt for idempotent per-team email
- MentorFile: add projectId (primary scope); mentorAssignmentId becomes nullable audit FK
- MentorChangeRequest: new model + status enum
- Migration hand-written with IF EXISTS guards (safe for docker-entrypoint retry)

prisma/migrations/20260522155652_multi_mentor_per_team/migration.sql

@@ -0,0 +1,78 @@
+-- Hand-written migration for PR8 (multi-mentor per team).
+--
+-- All DDL guarded with IF EXISTS / IF NOT EXISTS so the docker-entrypoint
+-- retry loop is safe to re-run. No regex (the 2026-05-07 prod incident was
+-- caused by Prisma 6 generating regex-based DDL that Postgres rejected).
+-- No BEGIN/COMMIT blocks — Prisma wraps the migration in a transaction.
+
+-- Phase 1: MentorAssignment — drop unique, add composite, add notification field
+ALTER TABLE "MentorAssignment" DROP CONSTRAINT IF EXISTS "MentorAssignment_projectId_key";
+DROP INDEX IF EXISTS "MentorAssignment_projectId_key";
+CREATE UNIQUE INDEX IF NOT EXISTS "MentorAssignment_projectId_mentorId_key"
+  ON "MentorAssignment"("projectId", "mentorId");
+CREATE INDEX IF NOT EXISTS "MentorAssignment_projectId_idx"
+  ON "MentorAssignment"("projectId");
+ALTER TABLE "MentorAssignment" ADD COLUMN IF NOT EXISTS "notificationSentAt" TIMESTAMP(3);
+
+-- Phase 2: MentorFile — re-scope to project (two-phase backfill)
+ALTER TABLE "MentorFile" ADD COLUMN IF NOT EXISTS "projectId" TEXT;
+UPDATE "MentorFile" mf
+   SET "projectId" = ma."projectId"
+  FROM "MentorAssignment" ma
+ WHERE mf."mentorAssignmentId" = ma."id"
+   AND mf."projectId" IS NULL;
+ALTER TABLE "MentorFile" ALTER COLUMN "projectId" SET NOT NULL;
+ALTER TABLE "MentorFile" DROP CONSTRAINT IF EXISTS "MentorFile_projectId_fkey";
+ALTER TABLE "MentorFile" ADD CONSTRAINT "MentorFile_projectId_fkey"
+  FOREIGN KEY ("projectId") REFERENCES "Project"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+CREATE INDEX IF NOT EXISTS "MentorFile_projectId_idx" ON "MentorFile"("projectId");
+
+-- Phase 2b: Make MentorFile.mentorAssignmentId nullable + switch its FK to SetNull
+ALTER TABLE "MentorFile" ALTER COLUMN "mentorAssignmentId" DROP NOT NULL;
+ALTER TABLE "MentorFile" DROP CONSTRAINT IF EXISTS "MentorFile_mentorAssignmentId_fkey";
+ALTER TABLE "MentorFile" ADD CONSTRAINT "MentorFile_mentorAssignmentId_fkey"
+  FOREIGN KEY ("mentorAssignmentId") REFERENCES "MentorAssignment"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- Phase 3: MentorChangeRequest table
+-- Postgres < 14 doesn't support CREATE TYPE ... IF NOT EXISTS, so wrap in a
+-- DO block that swallows duplicate_object errors (idempotent for re-runs).
+DO $$ BEGIN
+  CREATE TYPE "MentorChangeRequestStatus" AS ENUM ('PENDING', 'RESOLVED', 'DISMISSED');
+EXCEPTION
+  WHEN duplicate_object THEN NULL;
+END $$;
+
+CREATE TABLE IF NOT EXISTS "MentorChangeRequest" (
+  "id"                  TEXT NOT NULL,
+  "projectId"           TEXT NOT NULL,
+  "targetAssignmentId"  TEXT,
+  "requestedByUserId"   TEXT,
+  "reason"              TEXT NOT NULL,
+  "status"              "MentorChangeRequestStatus" NOT NULL DEFAULT 'PENDING',
+  "resolvedByUserId"    TEXT,
+  "resolvedAt"          TIMESTAMP(3),
+  "resolutionNote"      TEXT,
+  "createdAt"           TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  "updatedAt"           TIMESTAMP(3) NOT NULL,
+  CONSTRAINT "MentorChangeRequest_pkey" PRIMARY KEY ("id")
+);
+
+ALTER TABLE "MentorChangeRequest" DROP CONSTRAINT IF EXISTS "MentorChangeRequest_projectId_fkey";
+ALTER TABLE "MentorChangeRequest" ADD CONSTRAINT "MentorChangeRequest_projectId_fkey"
+  FOREIGN KEY ("projectId") REFERENCES "Project"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+ALTER TABLE "MentorChangeRequest" DROP CONSTRAINT IF EXISTS "MentorChangeRequest_targetAssignmentId_fkey";
+ALTER TABLE "MentorChangeRequest" ADD CONSTRAINT "MentorChangeRequest_targetAssignmentId_fkey"
+  FOREIGN KEY ("targetAssignmentId") REFERENCES "MentorAssignment"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+ALTER TABLE "MentorChangeRequest" DROP CONSTRAINT IF EXISTS "MentorChangeRequest_requestedByUserId_fkey";
+ALTER TABLE "MentorChangeRequest" ADD CONSTRAINT "MentorChangeRequest_requestedByUserId_fkey"
+  FOREIGN KEY ("requestedByUserId") REFERENCES "User"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+ALTER TABLE "MentorChangeRequest" DROP CONSTRAINT IF EXISTS "MentorChangeRequest_resolvedByUserId_fkey";
+ALTER TABLE "MentorChangeRequest" ADD CONSTRAINT "MentorChangeRequest_resolvedByUserId_fkey"
+  FOREIGN KEY ("resolvedByUserId") REFERENCES "User"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+CREATE INDEX IF NOT EXISTS "MentorChangeRequest_projectId_idx" ON "MentorChangeRequest"("projectId");
+CREATE INDEX IF NOT EXISTS "MentorChangeRequest_status_idx" ON "MentorChangeRequest"("status");
+CREATE INDEX IF NOT EXISTS "MentorChangeRequest_targetAssignmentId_idx" ON "MentorChangeRequest"("targetAssignmentId");