Shipwreck Proximity Alerts

Recent discovery events + couplings that happened NEAR a wreck.

For each entry in the public wreck catalog, finds discovery events from the last hours_back hours whose associated tensor snapshot (or metadata lat/lon if present) is within radius_km. The filter naturally narrows to EM-family events when em_only=True -- that is the only family whose physics maps to "buried iron" without ground-truth.

Intended consumers: shipwreck-prospecting clients on Phase I+ or Retainer plans who want a real-time "your wreck may be reacting" feed layered on top of their periodic survey reports.