Course base · Re-ingestion behaviour that preserves human edits

[lucaosti]

Sub-issue of #138. Depends on #164 (manual outline editing).

Goal

Define and implement what happens to an existing, reviewed outline when the underlying documents change.

The problem

A course has a reviewed, hand-corrected outline. The user then adds a document, replaces one, or deletes one. Today the behaviour is undefined, and both obvious options are wrong:

• Regenerate everything — discards every human correction
• Do nothing — the outline silently no longer reflects the material, and the student cannot tell

Proposed behaviour

Targeted invalidation rather than wholesale rebuild:

1. Detect which chapters and lessons are affected by the changed documents, via source_refs_json
2. Mark those items stale rather than deleting or regenerating them
3. Surface staleness in the review UI, stating why
4. Offer targeted regeneration per item, or accept-as-is
5. Never overwrite a human-edited item without explicit confirmation
6. For a genuinely new document with no overlap, propose new chapters rather than restructuring existing ones

Cases to handle

• Document added, covering new material
• Document added, overlapping existing chapters
• Document replaced with a newer version of the same material
• Document deleted — lessons sourced from it must not silently keep content whose source is gone
• Document reprocessed with a different parser (USE_DOCLING toggled), changing chunk ids
• All documents deleted

Tasks

• Staleness model on chapters and lessons, with a reason
• Detection linking document changes to affected items via source references
• Targeted regeneration preserving human edits, per Ladder · Migrate the seven modules that read QVAC_SERVICE_URL directly #160's edit provenance
• Review UI: staleness indicators, per-item regenerate, accept-as-is
• Handle deleted sources — lesson content citing a removed document must be flagged, not silently kept
• Tests for every case above

Acceptance criteria

• No document change silently discards human edits
• No document change silently leaves a stale outline unmarked
• Affected items identified precisely, not by regenerating the course
• Deleted sources flagged wherever their content is still referenced
• Every case above covered by a test

Note

Chunk ids changing under a parser switch is the subtle one: source references can appear valid while pointing at different text. Hash source content, not just ids.