fix(usage): compute credit balance from all-time usage, not a 30-day window

[AliaksandrNazaruk]

Problem

GET /api/usage computed balanceUsd = totalCredits − totalUsage, where totalCredits sums all-time confirmed/forwarded credit_deposits but totalUsage sums only the last-30-days usage_events (that query is windowed + .limit(1000) because it also feeds the history/breakdowns). So any spend older than 30 days is never subtracted and the balance is overstated — e.g. top up $100, spend $90 in January, return in March → balance shows ~$100 again (see #43).

Fix

Subtract an all-time usage total for the balance, computed via a DB-side aggregate (cost_usd.sum()), while keeping the existing 30-day query for the history/daily/module breakdowns. It falls back to the recent-window sum if PostgREST aggregates aren't available, so it can never overstate more than the current behavior:

let totalUsage = (events || []).reduce((s, e) => s + Number(e.cost_usd || 0), 0); // fallback
const { data: usageAgg, error: usageAggErr } = await admin
  .from("usage_events").select("total:cost_usd.sum()").eq("user_id", user.id).single();
const allTimeUsage = Number((usageAgg as { total?: number } | null)?.total);
if (!usageAggErr && Number.isFinite(allTimeUsage)) totalUsage = allTimeUsage;

Note

If you'd prefer a dedicated usage_total_cost(user_id) SQL function (the repo already uses .rpc(...) elsewhere) over the PostgREST aggregate, I'm happy to switch to a migration + RPC — just say the word. The aggregate keeps this PR code-only and safe by construction.

Fixes #43.