feat: surface async-migration batchSize / yieldDelay as config (currently hardcoded)

[Kpa-clawbot]

Context

Raised by Munger in PR #1735 (round-1 review). chunkedTxLastSeenBackfill takes batchSize and yieldDelay as parameters but they are hardcoded at the single call site (batchSize=5000, yieldDelay=100ms) and not surfaced for operator tuning.

Proposal

Surface both as config (config.json) with sane defaults:

"async_migrations": {
  "tx_last_seen_backfill": {
    "_comment_batch_size": "Rows per UPDATE chunk. Default 5000. Lower if reader latency spikes; higher if backfill is too slow.",
    "batch_size": 5000,
    "_comment_yield_delay_ms": "Milliseconds the writer rests between chunks. Default 100. Higher = more reader headroom, slower backfill.",
    "yield_delay_ms": 100
  }
}

Why

Operators see different DB sizes / I/O profiles; one tuning is not enough. Also lets us A/B in staging without code changes.

[Kpa-clawbot]

Severity: P3 — operability; defaults are fine, operators need a knob.

Claim: confirmed (forward-looking). PR #1735 introduces chunkedTxLastSeenBackfill(batchSize, yieldDelay) with hardcoded 5000 / 100ms at the call site. Different DB sizes / I/O profiles need different tuning.

Findings:

• (munger): One tuning won't fit MikroTik ARM64 prod, the staging x86 VM, and Cascadia-scale simultaneously. Config knob is cheap; the absence of it forces code edits + redeploy for tuning experiments.
• (kent-beck): Tests: one for default (5000/100), one for override path, one for invalid input rejection (negative/zero). Trivially testable.

Fix path:

1. Extend config.json schema with async_migrations.tx_last_seen_backfill.{batch_size, yield_delay_ms}.
2. Pipe through cmd/ingestor/db.go::Open config struct.
3. Replace the hardcoded call-site args.
4. Update config.example.json with both fields + _comment_ per CONFIG documentation rule.
   Effort: S.

Dup check: searched "async migration config batch_size" — none.

Blocker: depends on PR #1735 landing the chunked backfill surface.

Triage verdict: ready-to-fix

[Kpa-clawbot]

Auto-fix chain dispatched (cron v6.2, 2026-06-17 07:06 UTC). Will follow up with PR link or stop reason.