Reapply feat/labs migration (#1968)

[alanpeixinho]

Summary

Re-applies the labs migration reverted in #2057, restoring #1968 on main.

• Restores migration 0019_labs_builds_lab_tests_lab.py, the Labs model, and builds.lab / tests.lab FKs
• Restores query/ingester paths that read lab info from the lab column (with misc fallback where applicable)
• Auto-merged cleanly with [feat] AB comparison #2018 and fix(hardware): support cross-origin builds in revision selectors #2051 on hardware.py, tree.py, treeCommitsHistory.py, and schema.yml

Deployment notes

• Production: no prior migration state to unwind; migrate should apply 0019 normally
• Staging: if migration 0019 was rolled back to 0018 when Revert "feat/labs migration (#1968)" #2057 deployed, re-apply with python manage.py migrate kernelCI_app 0019 after deploy

Test plan

• CI green
• Staging manage.py migrate reports no pending/missing migrations
• Staging e2e suite passes
• Unblocks feat: labs listing #1985 (labs listing) and feat/backfill db #1986 (lab backfill)

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