fix(data): preserve K-line price precision

[mameikagou]

Summary

Fixes #218. Preserve provider OHLC precision instead of rounding every K-line price to four decimals.

Changes

• keep normalized OHLC values as floats without fixed-decimal rounding
• add regression coverage for low-priced instruments while retaining timestamp and volume behavior

Test plan

• 54 related pytest tests pass
• Ruff checks pass on changed files

API documentation

Not applicable; no routes or schemas changed.