IntegrityError deleting SensorLocation referenced by dailyavg2023 (missing FK in Django model)

Labels: bug, data-model, admin
Linked Sentry: SENSORSAFRICA-API-140 — https://code-for-africa.sentry.io/issues/7570022076/
Environment: production

## Summary
Deleting a SensorLocation in admin fails with:
update or delete on table "sensors_sensorlocation" violates foreign key constraint "fk_sensorlocation" on table "sensors_dailyavg2023"

## Root cause
`sensors_dailyavg2023` has a Postgres-level FK to `sensors_sensorlocation` that is not declared as a `ForeignKey` field in the Django model. Django's deletion collector doesn't know the relationship exists, so it doesn't cascade/null it before issuing the DELETE — Postgres correctly rejects it.

## Fix
Add the missing `ForeignKey` field to the Django model with an explicit `on_delete` (needs a product decision: CASCADE vs PROTECT vs SET_NULL — does deleting a location wipe historical daily averages, or should locations with historical data be undeletable?).

## To check
Whether `dailyavg2024`/`dailyavg2025` have the same gap. if so, fix should apply to all year-tables, not just 2023.