Add bounded observable Codewith history backfill

[andrei-hasna]

Problem

The current station backlog is approximately 9,792 Codewith JSONL files and 9.3 GB. A blind whole-history ingest is not operationally safe.

Acceptance

• Inventory/dry-run reports files, bytes, estimated rows, duplicates, and errors.
• Backfill supports batches, date/project ranges, checkpoints, resume, and cancellation.
• Backup and capacity checks run before mutation.
• One-file and date-range pilots are supported.
• Known IDs and counts can be verified before hosted sync.
• Error summaries do not expose transcript or credential content.

Todo: OPE2-00081

[andrei-hasna]

Closed by merged factory PR #51 (merge commit 5454297) after exact-head independent review GO and required checks.