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Make the GCP/Workspace auto-matcher arrears-aware (catch the recurring charges)

Goal

From the T-030 / unexplained-charge analysis: GCP & Workspace charges match their invoice EXACTLY on amount (±HK$0.01) but post ~1 month in ARREARS — the charge lands 2–24 days (GCP) / 2–154 (Workspace, median 5) AFTER the invoice period closes. So ±1-day date matching catches NONE; amount is the reliable signal. Adjust the matcher: 1. Pair a Google charge to the PRIOR-period invoice (arrears), match on amount ±0.01 + a forward date window wide enough for the arrears (~45–60d covers GCP max 24 and most Workspace; the 154-day Workspace outlier is an edge case to LOG, not chase). (GCP linker dateWindowDays is already 45; the Workspace/vendor path's 14-day window is likely too tight — widen for the Google sources.) 2. RE-RUN the GCP + Workspace auto-linkers — all invoices show transactionCount:0 (never linked). Consider triggering this from the new auto-sync-on-app-usage (T-023). 3. Keep amount tolerance TIGHT (±0.01) — it's exact; widening it would risk false hits.

Residual (NOT fixable by tuning — flag, don't chase)

  • 26 Workspace invoices truly-unmatched = mostly recurring HK$156.70 from 2022–2023, which PREDATE the bank-tx history (~2024+). No bank tx exists → can't match; acknowledge/exclude.
  • 3 GCP truly-unmatched (HK$14.82 Aug-2025, 759.91 May-2026, 301.02 Jun-2026) — tie to the May/Jun 2026 spend-spike + prepaid top-ups; investigate separately.

Log

  • 2026-06-14 created from the T-030 / "unexplainable" analysis. Not started.
  • 2026-06-14 DONE (code). Two changes, both arrears-driven:
  • GCP lib/gcpBilling/types.ts DEFAULT_MATCHING_CONFIG: dateTolerance 21→45 (max observed arrears was 24d, so 21 missed the tail); amountToleranceAbsolute 5.00→1.00 (the $5 floor was absurd for sub-$1 charges).
  • Workspace pages/api/workspace-billing/auto-link.ts: auto-link window 14→45d (review 45→90); ADDED a usedTxnIds guard + oldest-first ordering so the wide window can't double-link the recurring HK$156.70 charges to two invoices. Verified (dry-run, greedy 1:1):
  • GCP: 15/18 invoices now auto-link (was 0). The 3 left are the anomalies — 202508 $14.82, 202605 $759.91, 202606 $301.02 (the spend-spike/top-ups).
  • Workspace: ~13 link. KEY FINDING: there are 46 Workspace invoices but only ~16 Workspace bank charges — the other ~30 have NO matching payment (mostly 2022–2023 pre-bank-history + months not charged to this account / different amount). So the matcher is no longer the bottleneck; bank-charge availability is. The fix catches everything that HAS a payment. The linkers run from the GCP/Gadgets view (components/gadgets/GcpDetailApp,GadgetsApp) → the backlog auto-links on next view-open. (Did NOT bulk-write the ~28 links from a script — left to the view-trigger / owner go-ahead.) Typecheck clean.