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- Add published_only filter with admin-only override for unpublished posts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/infrastructure/di/providers.py

@@ -15,6 +15,8 @@ from app.application import (
 )
 from app.application.interfaces import TransactionManager
 from app.domain.repositories import PostRepository
+from app.infrastructure.auth import KeycloakAuthClient
+from app.infrastructure.config.settings import settings
 from app.infrastructure.database.connection import AsyncSessionLocal, engine
 from app.infrastructure.repositories.post import SQLAlchemyPostRepository
 
@@ -131,3 +133,12 @@ class UseCaseProvider(Provider):
             post_repo=post_repo,
             tx_manager=tx_manager,
         )
+
+
+class KeycloakProvider(Provider):
+    """Provider for Keycloak authentication client."""
+
+    @provide(scope=Scope.APP)
+    def get_keycloak_client(self) -> KeycloakAuthClient:
+        """Provide KeycloakAuthClient singleton."""
+        return KeycloakAuthClient(settings)