Targets refactor

[rawcomposition]

Replaces the legacy DigitalOcean serverless target species endpoint (slow, unreliable, scraped from eBird) with the new OpenBirding API. This also eliminates MongoDB storage for target data entirely, all target species are now fetched on-demand from OpenBirding.

Test plan

• Open any hotspot dialog (saved or unsaved) → targets load from OpenBirding
• Toggle "All Year" / trip range on hotspot targets → frequencies recompute client-side
• Visit targets page → region targets load directly, citation visible at bottom
• Expand a species on targets page → BestTargetHotspots calls POST endpoint
• Export KML → includes target data fetched from OpenBirding
• Change trip dates in settings → no redirect to import page, targets page reflects new dates
  automatically
• Run migration script manually: npx tsx backend/scripts/migrate-targets.ts