feat(trips): trip tracking with analytics, tag conversion, and transaction assignment

Adds trips table usage across API and UI: trip CRUD, per-trip analytics
(category/daily/merchant/tag/participant breakdowns), tag-to-trip
conversion, trip assignment via transaction overrides, and trip filter
in the transactions view. Recovered from working tree after local git
corruption; feature was already live via host-context Docker builds.

src/app/api/trips/[id]/transactions/route.ts

@@ -0,0 +1,15 @@
+import { NextRequest, NextResponse } from "next/server";
+import { getCurrentUser } from "@/lib/auth";
+import { assignTransactionsToTrip } from "@/lib/queries";
+
+export async function PATCH(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Forbidden" }, { status: 403 });
+  const { id } = await params;
+  const { transactionIds } = await req.json() as { transactionIds: number[] };
+  if (!Array.isArray(transactionIds) || !transactionIds.length) {
+    return NextResponse.json({ error: "transactionIds must be a non-empty array" }, { status: 400 });
+  }
+  await assignTransactionsToTrip(Number(id), transactionIds);
+  return NextResponse.json({ ok: true });
+}