Why

This PR addresses the following problem / context:

• Add legacy WeatherData support (model, migration, backfill, tests)
• Ensure WeatherData backfill can upsert cleanly with a PK even if table pre-exists

How

Implementation summary - the following was changed / added / removed:

• Added NMA_WeatherData model and corresponding unit tests
• Added Alembic migration to create the WeatherData table and enforce the OBJECTID PK if the table
  already exists
• Added WeatherData backfill script and wired it into the backfill orchestrator with
  BACKFILL_WEATHER_DATA

Notes

Any special considerations, workarounds, or follow-up work to note?

• WeatherData backfill uses ON CONFLICT (OBJECTID) and depends on the PK constraint to exist
• Set BACKFILL_WEATHER_DATA=false to skip in environments without the CSV loaded