fix(runs): fix B-tree index contention on actions table

[AdilFayyaz]

Why are the changes needed?

UpdateActionPhase writes updated_at/ended_at as NOW() on every call, causing B-tree right-hand page contention on those indexes under concurrent load (200ms+ lock-wait). The SQLite polling path in WatchActionUpdates also lacked a run_name filter, returning updates across all runs in a project instead of just the target run.

What changes were proposed in this pull request?

• Remove idx_actions_updated and idx_actions_ended to eliminate right-side B-tree hotspot contention on monotonically-increasing timestamp columns
• Replace four separate single-column indexes (idx_actions_org/project/domain/run_name) with one composite idx_actions_run_lookup(org, project, domain, run_name) to serve run-scoped queries with a single selective index scan
• Fix WatchActionUpdates SQLite polling query to include run_name in the WHERE clause, scoping results to the specific run
• Add migration 20260330_fix_action_indexes to drop the 6 stale indexes and trigger AutoMigrate to create the composite replacement

How was this patch tested?

• ran two psql queries to verify that the migration and the indexes were created correctly

1. Select id from migrations
   • Found the id: 20260330_fix_action_indexes
2. SELECT indexname FROM pg_indexes WHERE tablename = 'actions' ORDER BY indexname;
   • idx_actions_run_lookup is present (new composite index)
   • idx_actions_updated, idx_actions_ended, idx_actions_org, idx_actions_project, idx_actions_domain, idx_actions_run_name are all gone
3. ran hello.py from the sdk on the sandbox as a smoke test

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• I updated the documentation accordingly.
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Related PRs

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  • Flyte 2 WIP #6583
    • fix(runs): fix B-tree index contention on actions table 👈