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/merchants/route.ts

@@ -1,7 +1,10 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getMerchantSuggestions, getBankNames } from "@/lib/queries";
+import { getCurrentUser } from "@/lib/auth";
 
 export async function GET(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
   const search = req.nextUrl.searchParams.get("search");
   const type = req.nextUrl.searchParams.get("type");