Adds support for Brave Search API's freshness parameter to filter results
by discovery time:
- 'pd' - Past 24 hours
- 'pw' - Past week
- 'pm' - Past month
- 'py' - Past year
- 'YYYY-MM-DDtoYYYY-MM-DD' - Custom date range
Useful for cron jobs and monitoring tasks that need recent results.
Note: Perplexity provider ignores this parameter (Brave only).
---
🤖 AI-assisted: This PR was created with Claude (Opus). Lightly tested via
build script. The change follows existing patterns for optional parameters
(country, search_lang, ui_lang).
