[QTI] Backfill all non-raw AssessmentItems to QTI items #6007

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Overview

Run the one-time global backfill converting every legacy AssessmentItem to type='QTI' + raw_data XML via #6003's conversion, then remove the API-layer dual-read.

Complexity: Medium
Target branch: unstable

Context

• [QTI] Reusable legacy→QTI assessment-item conversion utility #6003 provides the reusable conversion.
• [QTI] Serve legacy assessment items as QTI through the API endpoint (dual-read) #6030 wires it into the endpoint as a transitional API dual-read.
• This issue makes that change permanent in the data.
• A data-only backfill — no schema change — over a large table.

The Change

• Backfill every legacy row to type='QTI' + raw_data XML via the [QTI] Reusable legacy→QTI assessment-item conversion utility #6003 conversion.
• Leave perseus_question rows untouched.
• Remove the API-layer dual-read ([QTI] Serve legacy assessment items as QTI through the API endpoint (dual-read) #6030) once the backfill completes.

Acceptance Criteria

• After the backfill, every legacy row is type='QTI' with raw_data XML; only perseus_question rows remain unconverted
• Rows that fail conversion are surfaced, not silently dropped
• The backfill is resumable and safe to re-run
• With the backfill complete, the API serves QTI directly without the dual-read
• Tests cover the backfill over a mixed fixture

AI usage

Architecture decided with the maintainer across an iterative session: blanket QTI type with the item XML in raw_data; XSD-authoritative validation across all sources; a legacy→QTI global migration with an API-layer dual-read; ricecooker upload delegating to the AssessmentItem serializer; and a Perseus custom-interaction contract confirmed against the QTI 3.0 specification. Claude mapped the existing publish/validation/ricecooker code, proposed the breakdown, and drafted each issue; the maintainer steered every decision and reviewed throughout.