docs(recipes): add sqlx persistence boundary

Document a provider-explicit SQLx persistence adapter for the SaaS tutorial without adding SQLx to hemx core or the workspace. The recipe keeps auth/session, CSRF, generated helpers, and Result handler mapping as the app boundary.

req: laws/002

req: laws/004

req: auth/001

req: auth/002

req: auth/004

req: examples/001

req: canonical_authoring/002

req: failure/004

README.md

@@ -76,7 +76,7 @@ and integrate at explicit boundaries. req: laws/002 req: auth/001
 
 - **Persistence:** use SQLx or another storage adapter in your application
   state/handlers. hemx should see ordinary domain values and generated UI
-  commands, not own the database layer.
+  commands, not own the database layer. See `docs/recipes/sqlx-persistence.md`.
 - **Auth/session:** use Axum/Tower extractors and middleware. Handlers may accept
   typed auth/session context and return ordinary HTTP failures or generated UI
   failures. req: auth/002
@@ -117,7 +117,9 @@ wire/runtime ABI; and advanced escape hatches that may remain integration-level.
   updates. Start here.
 - `examples/saas`: compile-tested v1 tutorial skeleton covering auth/session,
   CSRF-safe mutation, local persistence, generated swaps, page/push shape, plain
-  CSS, and one explicit island without provider-heavy platform scope.
+  CSS, and one explicit island without provider-heavy platform scope. The SQLx
+  persistence recipe in `docs/recipes/sqlx-persistence.md` shows the provider
+  boundary without moving SQL into core.
 - `examples/kanban`: advanced / north-star milestone boundary sketch. It may
   expose manual registry or render escape hatches while exploring product limits.
 - `examples/techdemo`: advanced integration demo with a leaf island and broader