docs(handover): add architecture map and takeover playbook

docs/product-engineering/request-lifecycle-flows.md

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+# Request Lifecycle Flows
+
+## 1. Auth Sign-In (Admin)
+
+1. Browser posts to `/api/auth/sign-in/email`.
+2. Route resolves `identifier` (email or username) to canonical email.
+3. Better Auth credential sign-in executes.
+4. Session cookie is set and user is redirected.
+
+Key files:
+- `apps/admin/src/app/login/login-form.tsx`
+- `apps/admin/src/app/api/auth/[...all]/route.ts`
+- `apps/admin/src/lib/auth/server.ts`
+
+## 2. Initial Owner Registration
+
+1. If no owner exists, `/welcome` renders owner sign-up mode.
+2. Sign-up request goes through auth route handler.
+3. New user is promoted to owner in transactional guard.
+4. Owner invariant is re-validated to enforce single owner.
+
+Key files:
+- `apps/admin/src/app/welcome/page.tsx`
+- `apps/admin/src/app/api/auth/[...all]/route.ts`
+- `apps/admin/src/lib/auth/server.ts`
+
+## 3. Media Upload
+
+1. Admin form posts multipart data to `/api/media/upload`.
+2. Metadata is validated and file is stored through selected provider.
+3. Media asset record is persisted with storage metadata.
+4. UI redirects back to media list with flash status query.
+
+Key files:
+- `apps/admin/src/components/media/media-upload-form.tsx`
+- `apps/admin/src/app/api/media/upload/route.ts`
+- `apps/admin/src/lib/media/storage.ts`
+- `packages/db/src/media-foundation.ts`
+
+## 4. Page Publish
+
+1. Admin submit on `/pages` calls server action.
+2. Page schema validates payload and persists.
+3. `published` status sets publication fields.
+4. Public app resolves slug and renders page if published.
+
+Key files:
+- `apps/admin/src/app/pages/page.tsx`
+- `packages/db/src/pages-navigation.ts`
+- `apps/web/src/app/[locale]/[slug]/page.tsx`
+
+## 5. Commission Status Transition
+
+1. Admin updates status from commission card form.
+2. Server action validates transition payload.
+3. DB update persists new status.
+4. Kanban view re-renders with updated column placement.
+
+Key files:
+- `apps/admin/src/app/commissions/page.tsx`
+- `packages/db/src/commissions.ts`