Fix PriorityQueue reopening existing databases

[MrQinlala]

Summary

• fix PriorityQueue initialization so it scans every PRAGMA table_info row before deciding whether the priority column exists
• keep creating the priority index with CREATE INDEX IF NOT EXISTS
• add a regression test for reopening an existing persisted priority queue database

Testing

go test ./... -run TestPriorityQueueReopenExistingDB -count=1
go test ./... -count=1

Both passed locally on Windows with Go 1.26.4 and GCC 14.1.0.

Fixes #5.

[codecov]

Codecov Report

❌ Patch coverage is 56.52174% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
priority_queue.go	56.52%	5 Missing and 5 partials ⚠️

📢 Thoughts on this report? Let us know!

[fahimfaisaal]

Hi @MrQinlala Thanks for the PR.

Can you fix the codecov as well?