Implements plan §14.5 Mode C work-queued chunked jobs for large
background operations (dump import, reshard backfill).
## Changes
### Core Implementation
- mode_c_coordinator.rs: Job coordination with claim/reclaim/heartbeat
- mode_c_worker/mod.rs: Worker loop for processing jobs
- mode_c_worker/acceptance_tests.rs: Full acceptance test suite
- reshard_chunking.rs: Shard-id range chunking for reshard backfill
### Database
- migrations/005_jobs_chunking.sql: Add chunking fields (parent_job_id,
chunk_index, total_chunks, created_at) with indexes
### Integration
- admin_endpoints.rs: Add ModeCWorker to AppState
- task_store: Updated to support chunking fields
- All test fixtures updated with new NewJob fields
## Acceptance Tests Pass
- 1 GB dump splits into 4× 256 MiB chunks; 3 pods claim in parallel
- Claim expires in 30s; another pod resumes at last_cursor
- HPA queue depth metric drives scaling (queue_depth > 10)
- Two concurrent dumps interleave without starvation
- Reshard backfill splits by shard-id range
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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