feat(ownership): Migrate assignee existence cache invalidation to use timestamp versioning on ownership

[shashjar]

Follow-up to #106108.

Closes ID-1238.

This PR extends the O(1) timestamp-based cache invalidation pattern (introduced in the above PR for the issue owners debounce cache) to the assignee existence check cache.

The invalidate_assignee_exists_cache method previously performed expensive fan-out O(n) cache invalidation. When CODEOWNERS or ownership rules changed, we would previously:

1. Query all groups in the project with recent events
2. Delete the assignee existence cache entry for each group individually (via batched cache.delete_many)

Before:

• Assignee existence cache stored a simple boolean
• On ownership change: delete cache entries for ALL active groups in the project (O(n))

After:

• Assignee existence cache stores (value, timestamp) tuple
• On ownership change, set the project-wide ownership_changed_at timestamp
• On cache read:
  • If cache is None --> query DB, cache result with timestamp
  • If cache is a tuple (value, timestamp), compare against ownership_changed_at
    • If ownership_changed_at is None or ownership_changed_at < cached_timestamp --> use cached value
    • If ownership_changed_at >= cached_timestamp --> ownership rules changed, re-query DB
  • If cache is old format (boolean) --> backwards compatibility, use value directly

Follow-Up

Left TODO in the code:

• Remove backwards compatibility logic for old boolean cache values 24 hrs after this PR is rolled out

[linear]

ID-1238 Cache for assignee existence check uses expensive fan-out invalidation

[shashjar]

bugbot run

[shashjar]

Adding a cache invalidation here (in the assign method) to expire stale value