feat: reset to system-calculated ranking order

Adds a 'Reset to system order' button per category (Startups and
Business Concepts). The button only appears when admins have
drag-reordered that category — otherwise there's nothing to reset.
Clicking it wipes that category's reorder history from the snapshot's
reordersJson via a new ranking.clearReorders mutation, after which
the dashboard re-initializes and falls back to the live composite
ranking (balanced/raw score, optionally blended with balanced pass
rate per the toggles).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/server/routers/ranking.ts

@@ -264,6 +264,32 @@ export const rankingRouter = router({
       return { ok: true }
     }),
 
+  /** Clear all manual drag-reorders for a snapshot, optionally filtered by
+   * category. The next dashboard render falls back to the system-computed
+   * order. Append-only history of reorders is wiped — that's the point. */
+  clearReorders: adminProcedure
+    .input(
+      z.object({
+        snapshotId: z.string(),
+        category: z.enum(['STARTUP', 'BUSINESS_CONCEPT']).optional(),
+      }),
+    )
+    .mutation(async ({ ctx, input }) => {
+      const snapshot = await ctx.prisma.rankingSnapshot.findUniqueOrThrow({
+        where: { id: input.snapshotId },
+        select: { reordersJson: true },
+      })
+      const existing = (snapshot.reordersJson as ReorderEvent[] | null) ?? []
+      const next = input.category
+        ? existing.filter((r) => r.category !== input.category)
+        : []
+      await ctx.prisma.rankingSnapshot.update({
+        where: { id: input.snapshotId },
+        data: { reordersJson: next as unknown as Prisma.InputJsonValue },
+      })
+      return { ok: true, cleared: existing.length - next.length }
+    }),
+
   /**
    * RANK-09 — Manual trigger for auto-rank (admin button on round detail page).
    * Reads ranking criteria from round configJson and executes quickRank.