The item form writes to-many edges against the related list; null-safe relationship cells; id coercion in admin routing

[borisno2]

Parent

#1127 — build spec 7 of the Prisma 8 architecture spec. Lands on the prisma-8 integration branch, not main.

What to build

The item form's to-many branch writes edges as writes against the related list (a FK update on the owning side, or a junction-row create/delete) instead of connect through an inverse field; every relationship cell and to-one read in the admin UI compiles against | null; admin routing parses ids through the contract-driven boundary coercion and 404s a malformed one; UI tests that touch data run on the Test context.

Acceptance criteria

• Selecting posts on a user's edit form updates each post's FK under the post list's access and reverts with the reason when denied
• A relationship cell renders a null to-one without error
• /admin/post/not-an-int on an integer-keyed list 404s
• No UI test hand-fakes context.db; pure-render tests fake at the prop level

Blocked by

• connect on the owning field, null as its counterpart, and the remove control's new branches #1153

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