add Post persistence: model, migration, eloquent + fake repo

PostModel maps posts table (id, user_id fk, title, body text,
created_at indexed). EloquentPostRepository: create, find,
findByUserId (desc by created_at), findRecent (limit, desc),
delete - chain via ::query() to keep larastan happy.
FakePostRepository sorts on read (defensive copy each return).
cascade-on-delete on user_id so removing a user nukes their
posts.

phpstan.neon suppresses staticMethod.dynamicCall under
app/*/Eloquent*Repository.php - phpstan-strict-rules flags
Eloquent's fluent builder idiom (Model::query()->orderBy())
because the static methods become instance calls mid-chain.
suppression scoped to repo files only so the rule still
applies elsewhere.

backend/phpstan.neon

@@ -7,6 +7,15 @@ parameters:
     treatPhpDocTypesAsCertain: false
     reportUnmatchedIgnoredErrors: false
 
+    ignoreErrors:
+        # Eloquent's fluent builder triggers staticMethod.dynamicCall in
+        # phpstan-strict-rules because Builder methods declared static on
+        # the model become instance calls after the first chain link. This
+        # is the documented Laravel idiom; suppress the false positive.
+        -
+            identifier: staticMethod.dynamicCall
+            path: app/*/Eloquent*Repository.php
+
     paths:
         - app