Three security-review findings on migration 002:
- pages.space_id and refs.space_id changed to NOT NULL + ON DELETE CASCADE
(was nullable + SET NULL, which allowed orphan rows surviving space deletion).
- pages.parent_id wrapped in composite FK (parent_id, space_id) -> pages(id, space_id)
to prevent cross-space parent linkage (same pattern as tasks.project_id in 001).
- idx_refs_external promoted to UNIQUE on (space_id, source_kind, external_id);
upsertByExternal now requires space_id and dedups per-space, not globally.
Added 3 regression tests covering composite FK rejection, CASCADE-on-space-delete,
and per-space dedup independence.
9dd944226d