Add briefing quality gates and ranked candidate selection

[walker1211]

Summary

• reject structured stories with missing, invalid, or cross-category source references
• derive source labels and timestamps from validated source article IDs
• support strong and weak category keywords with configurable weak-match thresholds
• rank category candidates by source priority, keyword strength, freshness, and near-duplicate penalties
• skip out-of-window RSS history before image enrichment and support per-feed item limits

Validation

• go test ./...
• go vet ./...
• go build ./cmds/news-briefing
• loaded a production-shaped configuration with strong/weak keywords and RSS item limits