feat(ops): job queue prioritization (M7)

[ArianAr]

Summary

Multi-job queue with rule priority and concurrent slot limit.

Closes #33

• Rule field priority (higher starts first when slots free)
• Engine queues jobs; GANTRY_MAX_CONCURRENT_JOBS / -max-concurrent-jobs (default 2)
• Cancel works for queued (remove from wait list) and active jobs
• Progress tab job queue panel + priority on rule form
• Fix: transfers use engine lifetime context (not HTTP request context)

Test

• go test ./...
• Frontend build

[ArianAr]

Review notes

• Priority is copied onto JobRun at enqueue time so later rule edits do not reshuffle already-queued work.
• Max concurrent jobs defaults to 2; dispatch is lock-protected and re-enters on job completion.
• Queued cancel removes from the in-memory wait list and marks the DB row cancelled (no cancel map entry until start).
• Transfers use engine lifetime context so finishing the HTTP start response no longer cancels in-flight jobs.

Approve when CI is green.