Fix valuation metadata loss and stock list updater safety

[cursor]

Bug and impact

• The ticker overview and historical report endpoints rebuilt ReportData with an incomplete field list, dropping persisted valuation metadata. The new valuation snapshot UI would show N/A/hidden values even though the valuation report stored fair value, method summary, bridge, sensitivity, and assumptions.
• The new Wikipedia stock-list updater accepted any non-empty parse, so a partial or wrong table parse could overwrite curated S&P 500 / NASDAQ-100 lists with a severely truncated universe.

Root cause

• backend/routers/tickers.py duplicated partial ReportData(...) mappings instead of preserving the full ReportService report dictionary shape.
• scripts/update_stock_lists.py treated non-empty fetched lists as valid without count or removal sanity checks.

Fix

• Added a shared ticker report mapper that passes through valuation metadata fields for latest and historical ticker report responses.
• Added stock-list safety validation for minimum counts and excessive removals, plus a --force override for deliberate large changes.
• Deferred the updater's pandas requirement to fetch-time so safety helpers remain importable/testable.

Validation

• PYTHONPATH=backend:. python3 -m unittest backend.tests.test_ticker_widgets_api scripts.test_update_stock_lists