feat(data): initialize local DuckDB on startup

[magic-alt]

Summary

• Initialize the configured local DuckDB market-data database during FastAPI startup.
• Reuse the resolved startup DuckDB path for local dataset listing, chart loading, and update writes.
• Return local store readiness metadata from /api/v2/local-data and clarify the Data page empty-state message.

Validation

• npm run build
• .venv/bin/python -m pytest tests/test_analysis_mvp.py tests/test_frontend_local_dev.py -v --tb=short
• git diff --check
• .venv/bin/python -m pytest tests/ -v --tb=short (passed with elevated permissions because the suite binds local ports and uses ProcessPoolExecutor)

Notes

• Startup initialization creates the DuckDB file/schema, but datasets remain empty until the first Update Local fetch persists OHLCV rows.