Fix: Prevent duplicate active alerts with unique index

[sentry]

This PR addresses the ActiveRecord::RecordInvalid: Validation failed: Address You already have an alert for that address error occurring in CuttlefishController#event.

Root Cause:
The issue stemmed from a race condition during alert creation, which allowed multiple active alerts for the same user and address to be saved. The Rails-level uniqueness validation (validates :address, uniqueness: { scope: %i[user_id unsubscribed] }) was insufficient to prevent this under concurrent requests, as there was no corresponding database-level unique index.

When Cuttlefish attempted to update an alert (e.g., last_delivered_at), the alert.update! call would trigger the Rails validation, find the other duplicate active alert, and raise ActiveRecord::RecordInvalid.

Solution:

1. Add Partial Unique Database Index: A new migration (20260813000000_add_unique_index_to_alerts.rb) has been added to create a partial unique index on alerts(address, user_id) where unsubscribed = false. This enforces uniqueness at the database level, preventing new duplicate active alerts from being created.
2. Clean Up Existing Duplicates: The migration includes a step to delete any existing duplicate active alerts, ensuring that only the oldest alert for a given (address, user_id) pair (where unsubscribed = false) remains before the index is added.
3. Update Rails Uniqueness Validation: The Alert model's validates :address, uniqueness: scope has been adjusted to { scope: :user_id } to align with the new partial database index. The unless: :unsubscribed? condition already handles the partial nature of the validation.
4. Graceful Error Handling in CuttlefishController: A rescue block has been added around alert.update! in CuttlefishController#event to catch ActiveRecord::RecordNotUnique and ActiveRecord::RecordInvalid exceptions. This ensures that if any residual duplicate active alerts cause a validation error during an update, the webhook call will still return a 200 OK, preventing Cuttlefish from retrying and potentially exacerbating the issue.

Fixes PLANNING-ALERTS-2

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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