[verified] Add BOT auto NPL (credit-quality) factor; deepen auto_credit theme

- auto_npl.py: parse BOT Gross NPLs by business (reportID=794); extract auto loan NPL (20,602 mn THB, 3.95% of NPLs, 2.06% of loans)
- /api/v1/themes now exposes auto_npl_pct + auto_npl_amount alongside car-sales volume
- 3 new tests; full suite 188 OK; live verified (themes shows auto_npl_pct 3.95)

docs/alternative-factor-source-research.md

@@ -16,6 +16,9 @@ The US EIA 3-2-1 crack spread is a **US** proxy; the user wants **Thai** refiner
 
 **Decision (final):** Use **Thai Oil (TOP) quarterly financial highlights** (`investor.thaioilgroup.com/en/financial-information/financial-highlights`) — real quarterly EBITDA/Net Profit/Sales of the largest Thai refinery (Million Baht), scraped from server-rendered HTML. Implemented in `backend/app/energy_thai.py`. This is Thai-specific and replaces both the US EIA crack spread and the Krungsri outlook projection. Frequency: **quarterly**. (Krungsri Research remains a qualitative backdrop; EIA is a US proxy — both superseded by TOP for the factor.)
 
+## Auto NPL (credit-quality) — added 2026-08-25
+BOT Gross NPLs by business type (`ReportPage.aspx?reportID=794`, FI_NP_003_S2). Extracts **auto-loan NPL** (20,602 mn THB, **3.95% of NPLs**, 2.06% of loans, Q2/2568). Deepens the auto_credit theme beyond new-car sales with credit quality. Implemented in `backend/app/auto_npl.py`; merged into the auto_credit theme read.
+