feat(jobsdb): multiconsumer datasets and flip

[atzoum]

Description

Adds the foundational schema and infrastructure for multi-consumer support in jobsdb. Today each jobsdb handle has a single implicit consumer: a job's latest status determines its state. This is a first step towards allowing jobs to be consumed independently by multiple consumers, each with its own status timeline per job.

The feature is opt-in per handle via WithMultiConsumer(). Single-consumer handles are completely unaffected — no schema changes, no query overhead.

An existing single-consumer jobsdb can be converted to multi-consumer at runtime. On first startup with WithMultiConsumer(), jobsdb upgrades any existing datasets to the multi-consumer schema. The upgrade is idempotent and crash-safe.

Linear Ticket

resolves PIPE-3058

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

---

This is part 3 of 3 in a stack made with GitButler:

•  3  feat(jobsdb): multiconsumer datasets and flip #7083 👈
•  2  chore(jobsdb): add consumer columns #7082
•  1  feat(jobsdb): introduce compaction min ds age #7077

[codecov]

Codecov Report

❌ Patch coverage is 88.71473% with 36 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (feat.mcJobsDBDDL@f0c14c3). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
jobsdb/jobsdb_dataset_ddl.go	20.83%	11 Missing and 8 partials ⚠️
jobsdb/jobsdb_multiconsumer.go	81.81%	6 Missing and 6 partials ⚠️
jobsdb/jobsdb_compaction.go	98.71%	1 Missing and 1 partial ⚠️
jobsdb/jobsdb_store.go	83.33%	1 Missing and 1 partial ⚠️
jobsdb/jobsdb_get.go	95.23%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##             feat.mcJobsDBDDL    #7083   +/-   ##
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  Coverage                    ?   79.50%           
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  Files                       ?      584           
  Lines                       ?    65444           
  Branches                    ?        0           
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  Hits                        ?    52033           
  Misses                      ?    10315           
  Partials                    ?     3096           

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