feat(02-01): add saveReorder mutation to ranking router

- Define ReorderEvent local type (category, orderedProjectIds, reorderedBy, reorderedAt)
- Add saveReorder adminProcedure accepting snapshotId, category, orderedProjectIds
- Append-only log: reads existing reordersJson, appends new event, persists full array
- Returns { ok: true } on success

src/server/routers/ranking.ts

@@ -12,6 +12,15 @@ import {
 import { logAudit } from '../utils/audit'
 import type { EvaluationConfig } from '@/types/competition-configs'
 
+// ─── Local Types ───────────────────────────────────────────────────────────────
+
+type ReorderEvent = {
+  category: 'STARTUP' | 'BUSINESS_CONCEPT'
+  orderedProjectIds: string[]
+  reorderedBy: string
+  reorderedAt: string
+}
+
 // ─── Zod Schemas ──────────────────────────────────────────────────────────────
 
 const ParsedRuleSchema = z.object({
@@ -204,6 +213,38 @@ export const rankingRouter = router({
       return snapshot
     }),
 
+  /** Persist admin drag-reorder to RankingSnapshot.reordersJson. Append-only — never overwrites old entries. DASH-02, DASH-03. */
+  saveReorder: adminProcedure
+    .input(
+      z.object({
+        snapshotId: z.string(),
+        category: z.enum(['STARTUP', 'BUSINESS_CONCEPT']),
+        orderedProjectIds: z.array(z.string()),
+      }),
+    )
+    .mutation(async ({ ctx, input }) => {
+      const snapshot = await ctx.prisma.rankingSnapshot.findUniqueOrThrow({
+        where: { id: input.snapshotId },
+        select: { reordersJson: true },
+      })
+
+      const existingReorders = (snapshot.reordersJson as ReorderEvent[] | null) ?? []
+
+      const newReorder: ReorderEvent = {
+        category: input.category,
+        orderedProjectIds: input.orderedProjectIds,
+        reorderedBy: ctx.user.id,
+        reorderedAt: new Date().toISOString(),
+      }
+
+      await ctx.prisma.rankingSnapshot.update({
+        where: { id: input.snapshotId },
+        data: { reordersJson: [...existingReorders, newReorder] as unknown as Prisma.InputJsonValue },
+      })
+
+      return { ok: true }
+    }),
+
   /**
    * RANK-09 — Manual trigger for auto-rank (admin button on round detail page).
    * Reads ranking criteria from round configJson and executes quickRank.