Persist repeated news co-mention candidate suggestions

[CrisSTEM]

Summary

• persist candidate suggestions for repeated news.co_mention mentions
• preserve the earliest first_seen_at while updating mention_count and last_seen_at
• keep reruns stable by updating the existing suggestion instead of creating duplicates
• add alert runtime coverage for positive detection, rerun suppression, noise filtering, catalog exclusions, and older mentions arriving later

Details

This implements the candidate suggestion path for alert rules using:

• event_key: news.co_mention
• condition.type: candidate_repeat_gte

The candidate extractor is intentionally conservative:

• excludes tracked entity names and aliases already present in the pack
• excludes binding query terms
• filters common generic/noise phrases
• only persists suggestions that repeat within the configured rule window

When the same candidate is seen again, the stored suggestion is updated in place:

• mention_count keeps the highest observed count
• first_seen_at keeps the earliest observed timestamp
• last_seen_at keeps the latest observed timestamp

Testing

• go test ./internal/alerts ./internal/storage
• go test ./...
• make ci

Deferred

• broader candidate extraction heuristics outside the current conservative rules
• additional candidate discovery event types beyond news.co_mention
• reviewer workflow / UI changes for candidate_suggestions

Closes #49