Add independent code review of the market data backend

[raunaksachdev]

Summary

• Fresh, independent code review of backend/app/market/ and its 73-test suite, written to planning/MARKET_DATA_REVIEW.md.
• Confirms all 7 issues from the prior archived review (planning/archive/MARKET_DATA_REVIEW.md) are genuinely fixed against current source.
• Verifies 73/73 tests pass, 91% coverage, lint clean.
• Surfaces a few new low-severity findings: a dormant falsy-0.0-timestamp edge case in PriceCache.update(), two test-quality gaps (test_exception_resilience doesn't inject a failure; test_custom_update_interval has a thin timing margin), and stale coverage figures in planning/MARKET_DATA_SUMMARY.md (84%/56% vs. actual 91%/94%).
• Empirically stress-tests the GBM simulator's correlation-matrix Cholesky decomposition across 4,083 ticker-subset combinations to confirm it never fails for this project's actual ticker universe.
• No blockers — the subsystem is ready to build on as-is.

Test plan

• uv run pytest -v — 73 passed
• uv run pytest --cov=app — 91% overall
• uv run ruff check app/ tests/ — clean

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