Move pricing rates into versioned pricing metadata

[SuvenSeo]

Goal

Move model pricing data out of codex_app_tracker.py and into versioned metadata such as pricing.json.

Why

Pricing changes over time. Reports should make it obvious which rate table was used.

Acceptance Criteria

• Pricing data lives in a small checked-in metadata file.
• Reports include pricing source date and table version.
• Tests cover cached input, input, and output pricing separately.
• README keeps the "estimate, not invoice" boundary clear.