fix(security+trips): auth/ownership on all API routes; trip analytics in AUD

Ten routes accepted requests with no getCurrentUser check (transactions/[id],
bulk, splits, tags-on-tx, splits/settle, statements/[id], tags, tags/[id],
merchants, participants/[id]/balance), and by-id routes did no ownership
check at all — any participant could read or modify another's data.

Adds canAccessTransactions() (owner via statement/direct, or split
participant), applies it to every transaction-scoped route, owner-scopes
statements/[id], and rescopes splits/settle in raw SQL so settlement only
touches splits the caller is party to.

Also: all trip analytics now sum COALESCE(amount_aud, amount) instead of raw
amount, matching every other analytics query — trip totals previously added
foreign-currency amounts to AUD ones unit-less.

And rules apply_split no longer delete+reinserts splits (which reset settled
flags on every run) — it upserts share_percent and removes only participants
no longer in the rule.

src/app/api/participants/[id]/balance/route.ts

@@ -1,5 +1,6 @@
 import { NextRequest, NextResponse } from "next/server";
 import { queryRaw } from "@/lib/db";
+import { getCurrentUser } from "@/lib/auth";
 
 interface BalanceRow {
   participant_id: number;
@@ -9,9 +10,11 @@ interface BalanceRow {
 }
 
 export async function GET(
-  _req: NextRequest,
+  req: NextRequest,
   { params }: { params: Promise<{ id: string }> }
 ) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const { id } = await params;
 
   const rows = await queryRaw<BalanceRow>(