Add support for job dependencies in pg boss

[klesgidis]

Summary

Add first-class job dependencies to pg-boss — a job can declare dependsOn parent jobs and won't be fetched until all parents complete. Enables fan-in and pipeline patterns without application-level orchestration.

What changed

• dependsOn option on send() / insert() — dependent jobs are inserted as blocked = true, invisible to fetch
• job_dependency table — normalized parent/child tracking with efficient indexes for both directions
• Automatic unblocking — CTE in completeJobs checks and unblocks children when all parents are done (no-op for queues without dependencies)
• blocking flag on parent jobs — lets the completion CTE short-circuit
• Updated job_i5 fetch index — adds AND NOT blocked at the index level, zero cost for non-dependent jobs
• getDependencies() / getDependents() — query the dependency graph
• Validation, cleanup, migration v30→v31 — all additive, no breaking changes

Usage

const jobA = await boss.send('process-data', { file: '1.csv' })
const jobB = await boss.send('process-data', { file: '2.csv' })

// jobC won't start until both jobA and jobB complete
const jobC = await boss.send('aggregate-results', { output: 'report.csv' }, {
  dependsOn: [
    { name: 'process-data', id: jobA },
    { name: 'process-data', id: jobB }
  ]
})

// query the graph
const parents = await boss.getDependencies('aggregate-results', jobC)
const children = await boss.getDependents('process-data', jobA)

Resolves #745

[coveralls]

coverage: 100.0%. remained the same
when pulling fb7a8e7 on klesgidis:master
into a827ec4 on timgit:master.