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.
Security review flagged that tasks.project_id could reference a project in
a different space. Added composite FK (project_id, space_id) -> projects(id, space_id)
with ON DELETE SET NULL (project_id) so a deleted project leaves the task in
its space with project_id NULL rather than orphaning into a NULL space.
Added two regression tests: cross-space FK rejection + cascade behavior.
