perf(keyword-metrics): batch freshness lookups

[hubab1]

Summary

• Batch and deduplicate SwiftData freshness reads once per selection pass.
• Fall back to the prior per-query lookup behavior if the bulk fetch fails.
• Preserve provider grouping, per-entry persistence, progress, cancellation, and failure isolation.

Verification

• KeywordMetricsServiceTests: 15 tests passed.
• Full suite: 185 tests in 22 suites passed.
• App build and launch verification passed.

Attribution

Adapted from PR #8 by @akshaynexus.