feat: add build worker that executes a single job

[Blackgaurd]

Extracts per-dataset build logic from _build_recursive() into a standalone execute_job() function in service/worker.py.

What changed

• New service/worker.py with execute_job(job, cancelled) -> JobResult
  • Generates valid timestamps, acquires per-dataset lock, checks existing timestamps
  • For each missing timestamp: fetches dep data (lookback vs single), runs builder subprocess, validates schema
  • Rows accumulated in memory, bulk-inserted only on full success (atomicity preserved)
  • Checks cancelled event between timestamps for early termination by failed siblings
  • Helper _fetch_dep_data() encapsulates dependency data fetching logic
• New tests/service/test_worker.py with 8 tests covering:
  • All timestamps exist (skip), missing timestamps built + inserted
  • Builder failure mid-range (no partial insert), cancelled event stops early
  • Lookback dep uses `get_rows_range`, no-lookback uses `get_rows_timestamps`
  • Missing dep data returns failure, no valid timestamps returns failure

Why

The worker is the execution layer in the scheduler/queue/worker refactor. It handles the "how to build one dataset" concern, separated from the "what order to build" concern (scheduler) and the "orchestrate the full plan" concern (orchestrator, next PR).

Benefit

Clean separation of build execution from dependency resolution. The orchestrator (PR 5) will call execute_job() for each job in the topological plan.

🤖 Generated with Claude Code

[Blackgaurd]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• refactor: replace _build_recursive with orchestrator #323 : 2 dependent PRs (#324 , #325 )
• feat: add build orchestrator #322
• feat: add build worker that executes a single job #321 👈 (View in Graphite)
• feat: add topological scheduler with Kahn's algorithm #320
• feat: add dependency graph collector with range expansion #319
• refactor: add job descriptor and build plan types #318
• main

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