test(workbook): qualify complete legacy migration

[rudycelekli]

What & why

Snapshot every matching legacy lead during workbook migration. Read source pages of 500 and flush row batches of 50, then commit the complete snapshot and storage marker together. A later-page failure rolls back so retry can finish.

This owned-fork PR runs the existing four-job qualification workflow for the candidate.

Verification

• Exact main with final regressions: four fail and three controls pass. Local candidate migration/cell-scope suite: 46 pass.
• Actual production route, WorkspaceCtx lead factory, LeadDB producer and migrated SQLite: main truncates 501/1,001 leads to 500 and subsequent calls report already_migrated=500; candidate keeps all rows and tail enrichment, respects filters and preserves repeated-call behavior.
• Independent cold review additionally ran the real route on 500 and 1,501 leads (four source pages): all IDs, positions, tail receipts, filters and repeated migration passed.
• Source/test hashes and all receipts are retained in the private campaign record. All four hosted jobs passed on the signed candidate commit.

Boundaries

No proof of a consistent snapshot while source leads change concurrently, or repair of previously completed partial migrations. HTTP auth, PostgreSQL and frontend qualification belong to the existing hosted workflow. The existing enrichment map retains its memory behavior.

Checklist

• Commit carries Rudy's DCO sign-off
• Scope is one workbook migration correctness fix
• Full existing four-job CI passed

Hosted qualification

Existing four-job workflow passed at ada5e7b2d9b90dd2eb61028adf88fa0584641868: SQLite (2,099 pass, 134 skip), PostgreSQL integrity/tenancy (18 pass), frontend lint/tests/build, and mounted browser regressions (3 pass). No CI workflow change.

[rudycelekli]

Superseded by owned-fork validation #6 using the identical reviewed tree with an explicit GitHub-verified SSH signature. This original head has a DCO sign-off but no cryptographic signature; its qualification receipts remain preserved. No upstream publication.