feat(shared): replace settle buttons with payment ledger

- New split_payments table records actual payments between participants
- Balance = total split obligations - total payments (splits never marked settled)
- Record Payment modal per participant: direction toggle, amount pre-filled with balance, date, notes
- Payment history inline on each balance card with +/- display and delete
- Per-transaction Settle button removed; Action column removed from shared table
- Splits always show the true cost breakdown regardless of payment state

src/app/api/split-payments/route.ts

@@ -0,0 +1,88 @@
+import { NextRequest, NextResponse } from "next/server";
+import { getCurrentUser } from "@/lib/auth";
+import { queryRaw } from "@/lib/db";
+import { prisma } from "@/lib/db";
+
+export async function GET(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
+
+  const sp = req.nextUrl.searchParams;
+  const participantId = sp.get("participant_id");
+
+  // Return payment history between current user and a participant
+  const rows = await queryRaw<{
+    id: number;
+    from_participant_id: number;
+    from_name: string;
+    to_participant_id: number;
+    to_name: string;
+    amount: number;
+    payment_date: string;
+    notes: string | null;
+    linked_transaction_id: number | null;
+    created_at: string;
+  }>(
+    `SELECT sp.id, sp.from_participant_id, pf.name as from_name,
+            sp.to_participant_id, pt.name as to_name,
+            sp.amount, sp.payment_date, sp.notes,
+            sp.linked_transaction_id, sp.created_at
+     FROM split_payments sp
+     JOIN participants pf ON pf.id = sp.from_participant_id
+     JOIN participants pt ON pt.id = sp.to_participant_id
+     WHERE (sp.from_participant_id = $1 OR sp.to_participant_id = $1)
+       AND (sp.from_participant_id = $2 OR sp.to_participant_id = $2)
+     ORDER BY sp.payment_date DESC, sp.created_at DESC`,
+    [user.id, participantId ? Number(participantId) : user.id]
+  );
+
+  return NextResponse.json(rows);
+}
+
+export async function POST(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
+
+  const body = await req.json() as {
+    from_participant_id: number;
+    to_participant_id: number;
+    amount: number;
+    payment_date: string;
+    notes?: string;
+    linked_transaction_id?: number;
+  };
+
+  const { from_participant_id, to_participant_id, amount, payment_date, notes, linked_transaction_id } = body;
+
+  if (!from_participant_id || !to_participant_id || !amount || !payment_date) {
+    return NextResponse.json({ error: "Missing required fields" }, { status: 400 });
+  }
+  if (amount <= 0) {
+    return NextResponse.json({ error: "Amount must be positive" }, { status: 400 });
+  }
+
+  const payment = await prisma.split_payments.create({
+    data: {
+      from_participant_id,
+      to_participant_id,
+      amount,
+      payment_date: new Date(payment_date),
+      notes: notes || null,
+      linked_transaction_id: linked_transaction_id || null,
+    },
+  });
+
+  return NextResponse.json(payment);
+}
+
+export async function DELETE(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
+
+  const sp = req.nextUrl.searchParams;
+  const id = Number(sp.get("id"));
+  if (!id) return NextResponse.json({ error: "id required" }, { status: 400 });
+
+  await prisma.split_payments.delete({ where: { id } });
+  return NextResponse.json({ ok: true });
+}