feat: Add indicator coverage monitoring to gaps page

- Add "Indicator Coverage" tab alongside existing "Gap Detection" tab
- Display coverage summary with total/complete/incomplete combinations
- Show detailed coverage table with progress bars and status badges
- Add backfill buttons for individual symbol/interval pairs
- Add bulk backfill option for all incomplete indicators
- Include filter and search functionality for both tabs
- Show missing indicator counts and details per combination
- Real-time refresh capabilities for both gaps and indicators
- Maintain all existing gap detection functionality
- Provide visual progress bars showing coverage percentages
- Support batch operations with confirmation dialogs

This integrates indicator coverage monitoring into the existing gaps
interface, providing a unified data quality dashboard for monitoring
both OHLCV gaps and technical indicator completeness.

.gitignore

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 # Custom rules (everything added below won't be overriden by 'Generate .gitignore File' if you use 'Update' option)
 
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