fix(deriver): keep active work unit claims alive

[PraveenKumarSridhar]

Summary

• heartbeat exact ActiveQueueSession claims while long-running queue work executes
• fail closed when ownership is lost or heartbeat database operations stall
• require exact claim ownership before marking queue items processed or errored
• emit queue.empty only after a clean queue drain
• cover dream, summary, and representation processing races

Closes #794

Test plan

• 112 passed — deriver and DB-resilience suites
• uv run ruff check src/ tests/
• uv run basedpyright
• uv run pre-commit run --files src/deriver/queue_manager.py tests/deriver/test_queue_processing.py

The full pytest run stops at TestDocumentCRUD::test_duplicate_rejection_reinforces_existing when no OpenAI API key is configured. The identical failure was reproduced on clean upstream/main.

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