fix: change summary_messages/lineage/context_items FK from RESTRICT to CASCADE/SET NULL

[Eugene9D]

Problem

summary_messages.message_id uses ON DELETE RESTRICT, but SQLite defaults to PRAGMA foreign_keys=0. This means message deletes silently succeed and leave orphaned rows in summary_messages pointing at non-existent message_ids.

Same issue affects summary_lineage.parent_summary_id and context_items.message_id/summary_id.

On a production database (~607MB, 58K messages, 1098 summaries), orphaned summary_messages rows accumulated silently over weeks.

Fix

Table	Column	Before	After	Rationale
summary_messages	message_id	RESTRICT	CASCADE	Link row has no meaning without source message
summary_lineage	parent_summary_id	RESTRICT	CASCADE	Lineage row is meaningless without parent
context_items	message_id	RESTRICT	SET NULL	Preserve context row, null out dead ref
context_items	summary_id	RESTRICT	SET NULL	Preserve context row, null out dead ref

Migration for existing databases

Adds migrateForeignKeyCascade() — idempotent table rebuild that:

1. Checks current FK action via PRAGMA foreign_key_list
2. Only rebuilds if RESTRICT is still present
3. Filters orphans during INSERT INTO ... SELECT (cleans existing orphans)
4. Rebuilds indexes

Testing

Applied locally on a 607MB production LCM database:

• Before: orphaned summary_messages rows accumulating
• After: 0 orphans, all 1098 summaries intact, CASCADE verified via PRAGMA foreign_key_list