feat(backtest): runnable with partial factor coverage + oldest-history default (owner rule)

Owner's rule: backtest must run as soon as there's enough data to estimate an
investment — it must NOT be blocked just because some sources lack deep PIT
history. Scoring is deliberately flexible (a theme uses whatever subset of
factors was knowable that day).

- backtest_readiness: readiness = usable window (price + Siamchart + >=1 factor),
  not all-factors-present. Missing factors still reported (transparency) but no
  longer block the run. recommended_start = oldest executable price (oldest
  history the system holds); recommended_end = last complete trading day.
- pit_scorer.theme_surprise_report: flexible — skips factors not released by
  as_of; blocked only when NO factor has a value. pit_meta.partial_pit reflects
  themes scored from a partial factor subset.
- Verified end-to-end: readiness ready=true (recommended 2024-01-03 -> 2026-08-29);
  POST /api/v1/backtest/run default window returns 201 full result (1M -> final
  equity 1,117,243.95), no 400 from missing factors.
- test_backtest_readiness updated to earliest-runnable semantics; full suite 369 green.

docs/engineering-log/2026-08-29-data-source-expansion-and-ui-fix.md

@@ -88,3 +88,24 @@ needs-deeper-capture and left as a decision for the owner.
 - Note: a **concurrent process** also landed `thai_trade.py` (external-sector
   exports/imports/current-account) and external_* factors mid-session; its 3
   initially-broken tests were fixed to reach the 360-green baseline here.
+
+## Flexible PIT backtest (owner rule — 2026-08-30)
+Owner clarified the backtest contract: it must run as soon as there is enough
+data to estimate an investment, NOT only when EVERY factor has PIT history.
+Some sources may not provide deep history — scoring is deliberately flexible
+(a theme scores from whatever subset of factors was knowable that day).
+
+Implemented (commit 2026-08-30):
+- `backtest_readiness.evaluate_readiness`: readiness is now a *usable* window
+  (price + Siamchart + >=1 released factor), not all-factors-present. Missing
+  factors are still reported in `missing` for transparency but no longer block.
+  `recommended_start` = oldest executable price (the oldest history held);
+  `recommended_end` = last complete trading day (yesterday Bangkok).
+- `pit_scorer.theme_surprise_report`: flexible — skips any factor with no release
+  by `as_of` (no blanket theme block); `blocked` only when NO factor has a value.
+  `pit_meta.partial_pit` reflects themes that scored from a partial subset.
+- Verified: `/api/v1/backtest/readiness` → ready=true, recommended 2024-01-03 →
+  2026-08-29. `POST /api/v1/backtest/run` (default window) → 201 full result
+  (final_equity 1,117,243.95 on 1M), no 400 from missing factors.
+- Tests: test_backtest_readiness updated to earliest-runnable semantics; full
+  suite 369 green.