feat(uptime): Add UptimeResponseCapture to cleanup infrastructure

[wedamija]

Register UptimeResponseCapture with the deletions framework so that old response captures are cleaned up. Uses ModelDeletionTask to ensure the custom delete() method is called, which properly removes the associated File and its storage blobs.

Also adds an index on date_added for cleanup queries.

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/uptime/migrations/0052_add_date_added_index_to_uptimeresponsecapture.py

for 0052_add_date_added_index_to_uptimeresponsecapture in uptime

--
-- Create index uptime_upti_date_ad_d591fd_idx on field(s) date_added of model uptimeresponsecapture
--
CREATE INDEX CONCURRENTLY "uptime_upti_date_ad_d591fd_idx" ON "uptime_uptimeresponsecapture" ("date_added");

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Test bypasses established fixture pattern for consistency

Low Severity

The test creates an UptimeSubscription directly via UptimeSubscription.objects.create() instead of using the available self.create_uptime_subscription() fixture method. Other tests in the same file follow the fixture pattern (e.g., self.create_project(), self.create_group()). Using the fixture would be consistent with the codebase's test patterns and provide default values, making the test more maintainable if the model's required fields change.