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- Add published_only filter with admin-only override for unpublished posts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/domain/repositories/post.py

@@ -15,17 +15,31 @@ class PostRepository(Repository[Post]):
         ...
 
     @abstractmethod
-    async def get_by_author(self, author_id: str) -> list[Post]:
+    async def get_by_author(
+        self,
+        author_id: str,
+        limit: int | None = None,
+        offset: int | None = None,
+    ) -> list[Post]:
         """Get all posts by author."""
         ...
 
     @abstractmethod
-    async def get_published(self) -> list[Post]:
+    async def get_published(
+        self,
+        limit: int | None = None,
+        offset: int | None = None,
+    ) -> list[Post]:
         """Get all published posts."""
         ...
 
     @abstractmethod
-    async def get_by_tag(self, tag: str) -> list[Post]:
+    async def get_by_tag(
+        self,
+        tag: str,
+        limit: int | None = None,
+        offset: int | None = None,
+    ) -> list[Post]:
         """Get posts by tag."""
         ...
 
@@ -35,6 +49,11 @@ class PostRepository(Repository[Post]):
         ...
 
     @abstractmethod
-    async def search(self, query: str) -> list[Post]:
+    async def search(
+        self,
+        query: str,
+        limit: int | None = None,
+        offset: int | None = None,
+    ) -> list[Post]:
         """Search posts by query string."""
         ...