feat(finalist): unenroll reverses round membership + confirmation

Adds finalist.unenroll(projectId, roundId) which deletes the
FinalistConfirmation (cascading AttendingMember/FlightDetail/
VisaApplication/MemberLunchPick) and the LIVE_FINAL ProjectRoundState,
then logs a FINALIST_UNENROLL audit entry. Safe no-op when no rows exist.
Tests cover ADMIN_CONFIRM enrolled teardown and the no-rows path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/server/routers/finalist.ts

@@ -1273,4 +1273,49 @@ export const finalistRouter = router({
 
       return { enrolled, emailed, adminConfirmed, skipped }
     }),
+
+  /**
+   * Reverse enrollment: removes a project from the LIVE_FINAL round and
+   * deletes its FinalistConfirmation (cascade removes AttendingMember,
+   * FlightDetail, VisaApplication, and MemberLunchPick rows).
+   *
+   * Mentor assignments (tied to the MENTORING round) are intentionally
+   * left untouched. Safe to call even if the project was never enrolled
+   * (deleteMany is a no-op when no rows match).
+   */
+  unenroll: adminProcedure
+    .input(
+      z.object({
+        projectId: z.string(),
+        roundId: z.string(), // the LIVE_FINAL round
+      }),
+    )
+    .mutation(async ({ ctx, input }) => {
+      // Step 1: Delete the FinalistConfirmation (cascade removes AttendingMember
+      // / FlightDetail / VisaApplication / MemberLunchPick).
+      // deleteMany is no-op-safe when no row exists.
+      await ctx.prisma.finalistConfirmation.deleteMany({
+        where: { projectId: input.projectId },
+      })
+
+      // Step 2: Delete the LIVE_FINAL ProjectRoundState.
+      await ctx.prisma.projectRoundState.deleteMany({
+        where: { projectId: input.projectId, roundId: input.roundId },
+      })
+
+      // Step 3: Audit log
+      await logAudit({
+        prisma: ctx.prisma,
+        userId: ctx.user.id,
+        action: 'FINALIST_UNENROLL',
+        entityType: 'Project',
+        entityId: input.projectId,
+        detailsJson: {
+          projectId: input.projectId,
+          roundId: input.roundId,
+        },
+      })
+
+      return { ok: true }
+    }),
 })