PriceObservation has zero mutation methods, enforced by a reflection
test. Fixed a real bug found while writing the EF config: Product ->
PriceObservation was wired Cascade (copy-paste), which would let
deleting a Product silently destroy its price history. Changed to
Restrict.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Purchase (aggregate root) and PurchaseItem per AGENTS.md §5.6.
AddItem composition + tightened PurchaseItem ctor deferred to
CW-STORY-05.4, mirroring the CW-STORY-03.1/03.3 split.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Retailer/StoreLocation per AGENTS.md §5.5. DEC-011: store tracking is
optional everywhere (nullable FKs), so recording a purchase never
requires picking a store first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Brand, Product (aggregate root for ProductIdentifier), ProductIdentifier,
and HouseholdProductPreference, per AGENTS.md §5.2/5.3. Built ahead of
MVP-tagged epics per DEC-010 (explicit founder direction to work epics
in strict numeric order).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>