feat(auth): implement Keycloak authentication with RBAC and pagination

Major changes:
- Add Keycloak integration via token introspection endpoint
- Implement RBAC system with roles: admin, user, guest
- Add role-based permissions for post operations
- Add pagination support (default limit: 10) to list endpoints
- Add published_only filter with admin-only override for unpublished posts

Security improvements:
- Remove hardcoded default secrets (SECRET_KEY, KEYCLOAK_CLIENT_SECRET)
- Update .env.example with proper security placeholders
- Add comprehensive RBAC unit tests

Infrastructure:
- Add httpx dependency for HTTP client
- Add KeycloakAuthClient with token caching (TTL: 60s)
- Add role-based dependencies (RequireAdmin, RequireUser, etc.)
- Update DI container with Keycloak provider

Endpoints updated:
- GET /posts: filter by published status (admin can see all)
- Add pagination params (limit, offset) to list endpoints
- Enforce RBAC on post operations

Tests:
- Add 16 auth infrastructure tests
- Add 13 RBAC role tests
- Update existing tests for new required settings

Breaking changes:
- SECRET_KEY and KEYCLOAK_CLIENT_SECRET now required (no defaults)

app/application/use_cases/list_posts.py

@@ -22,24 +22,45 @@ class ListPostsUseCase:
         posts = await self._post_repo.get_all()
         return [self._map_to_dto(post) for post in posts]
 
-    async def published_posts(self) -> list[PostResponseDTO]:
+    async def published_posts(
+        self,
+        limit: int | None = None,
+        offset: int | None = None,
+    ) -> list[PostResponseDTO]:
         """Get all published posts."""
-        posts = await self._post_repo.get_published()
+        posts = await self._post_repo.get_published(limit=limit, offset=offset)
         return [self._map_to_dto(post) for post in posts]
 
-    async def by_author(self, author_id: str) -> list[PostResponseDTO]:
+    async def by_author(
+        self,
+        author_id: str,
+        limit: int | None = None,
+        offset: int | None = None,
+    ) -> list[PostResponseDTO]:
         """Get posts by author."""
-        posts = await self._post_repo.get_by_author(author_id)
+        posts = await self._post_repo.get_by_author(
+            author_id, limit=limit, offset=offset
+        )
         return [self._map_to_dto(post) for post in posts]
 
-    async def by_tag(self, tag: str) -> list[PostResponseDTO]:
+    async def by_tag(
+        self,
+        tag: str,
+        limit: int | None = None,
+        offset: int | None = None,
+    ) -> list[PostResponseDTO]:
         """Get posts by tag."""
-        posts = await self._post_repo.get_by_tag(tag)
+        posts = await self._post_repo.get_by_tag(tag, limit=limit, offset=offset)
         return [self._map_to_dto(post) for post in posts]
 
-    async def search(self, query: str) -> list[PostResponseDTO]:
+    async def search(
+        self,
+        query: str,
+        limit: int | None = None,
+        offset: int | None = None,
+    ) -> list[PostResponseDTO]:
         """Search posts."""
-        posts = await self._post_repo.search(query)
+        posts = await self._post_repo.search(query, limit=limit, offset=offset)
         return [self._map_to_dto(post) for post in posts]
 
     def _map_to_dto(self, post: Post) -> PostResponseDTO: