Fix 500 when a toggle or refresh clears the chart cache mid-request

[posthog]

Problem

• A user hit four 500s in 0.11 s on the demo dashboard: a chart request read a cache entry another request had just cleared.
• FastHTML runs the sync handlers in a thread pool, so requests overlap. The lazy chart route /batch/{batch_name}/chart/{chart_index} indexed the shared df_cache, stats_cache, and chart_cache dicts directly and used a check-then-use on chart_cache.
• A toggle (chart_cache.clear()), a refresh (deletes one batch), or a search (clear_batch_cache) landing mid-request removes the key, so the route raises KeyError and returns a 500 instead of a chart.
• It recurs for anyone who flips a display toggle while a batch page is still loading its charts.

Changes

• Read each cache once into a local and rebuild on a miss, so the request keeps its own references that a concurrent clear cannot remove — the pattern the index and anomaly-list routes already use.
• Replace the chart_cache check-then-use with setdefault plus a local fig; the render now uses that local instead of indexing the dict a second time.
• calculate_metric_stats accepts an explicit dataframe and returns the stats, so the caller never re-reads a dict that may have been cleared.
• Add a regression test that clears every cache during chart creation and asserts the route still renders. It reproduces the original KeyError on the old code.

Test plan

• pytest tests/test_dashboard.py — 9 passed
• ruff check and ruff format clean on the changed files
• Confirmed the new test fails with KeyError against the pre-fix render path and passes after

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[github-actions]

📊 Test Coverage Report

Coverage: 56% (yellow)

✅ Coverage maintained or improved!

💡 See detailed coverage report in the tests README