feat(merchants): scatter plot, merchant profiles, and per-merchant transaction history

- /merchants page with spend-vs-frequency scatter chart (click to open profile)
- Merchant profile drawer: stats, monthly trend line, full transaction history
- /api/analytics/merchants: split-adjusted merchant aggregates + monthly trends
- /api/analytics/merchants/[merchant]: per-merchant transaction list
- Add Merchants nav item to sidebar

src/app/api/analytics/merchants/[merchant]/route.ts

@@ -0,0 +1,53 @@
+import { NextRequest, NextResponse } from "next/server";
+import { getCurrentUser } from "@/lib/auth";
+import { queryRaw } from "@/lib/db";
+
+export async function GET(
+  req: NextRequest,
+  { params }: { params: Promise<{ merchant: string }> }
+) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+
+  const { merchant } = await params;
+  const decoded = decodeURIComponent(merchant);
+
+  const transactions = await queryRaw<{
+    id: number;
+    transaction_date: string;
+    description: string;
+    amount: number;
+    amount_aud: number | null;
+    my_amount: number;
+    transaction_type: string;
+    category: string;
+    bank_name: string;
+    statement_id: number;
+  }>(`
+    SELECT
+      t.id,
+      t.transaction_date::text,
+      t.description,
+      t.amount,
+      t.amount_aud,
+      CASE
+        WHEN ts.share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * ts.share_percent / 100
+        ELSE COALESCE(t.amount_aud, t.amount)
+      END::numeric(10,2) as my_amount,
+      t.transaction_type,
+      COALESCE(o.category_override, t.category) as category,
+      s.bank_name,
+      t.statement_id
+    FROM transactions t
+    JOIN statements s ON s.id = t.statement_id
+    LEFT JOIN transaction_overrides o ON o.transaction_id = t.id
+    LEFT JOIN transaction_splits ts ON ts.transaction_id = t.id AND ts.participant_id = $1
+    WHERE s.owner_id = $1
+      AND t.transaction_type IN ('debit', 'fee')
+      AND COALESCE(o.merchant_normalized, t.merchant_normalized, t.merchant_name, t.description) = $2
+    ORDER BY t.transaction_date DESC
+    LIMIT 500
+  `, [user.id, decoded]);
+
+  return NextResponse.json({ transactions });
+}