Add database indexes for PR lookup and queue queries

[arferreira]

The pull_requests table had zero indexes despite being queried heavily. Adds two indexes via migration:

• `idx_pr_repo_number_status` on (repo_owner, repo_name, pr_number, status) for the primary PR lookup pattern used by find_queued_pr and find_active_pr.
• `idx_pr_status_priority_queued` on (status, priority, queued_at) for queue ordering used by get_queue, get_next_queued, and the dashboard active PR query.

[arferreira]

@fila ship

[fila-ci]

Commit f1ed531 has been added to the merge queue by @arferreira.

View queue

[fila-ci]

#29 merged into main (b83b9ab)