Add DB index for feature-flags lookup hot path [b#097]

[greatest0fallt1me]

Description

This is a backend issue for the GrantFox FWC26 campaign. EXPLAIN-verified index on the hot /api/feature-flags filter columns; migration + rollback.

Requirements and Context

• Implement per the description above
• Add focused tests for the change
• Document any API/visible changes
• Adhere to repo's lint and code style
• Must be secure, tested, and documented

Suggested Execution

1. Fork the repo and create a branch
2. Implement changes touching:
   • migrations/feature-flags_index.sql
   • src/routes/feature-flags.ts
3. Test and commit; open a PR with Closes #<issue-number> in the body

Guidelines

• Minimum 90% test coverage on changed lines
• Input validation at the boundary; standardized error envelope
• Structured logging with correlation IDs
• Clear documentation and inline comments
• Timeframe: 96 hours