Add idempotency key for addons table

[jakubno]

Note

Adds nullable idempotency_key to addons with a unique partial index and updates the Addon model accordingly.

• Database:
  • Adds nullable idempotency_key column to public.addons.
  • Creates unique partial index addons_idempotency_key_uidx on idempotency_key when not null.
  • Includes down migration to drop index and column.
• Models:
  • Updates Addon struct in packages/db/queries/models.go to include IdempotencyKey *string.

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[linear]

ENG-3219 Addons idempotency