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- Add published_only filter with admin-only override for unpublished posts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/auth/models.py

@@ -0,0 +1,34 @@
+"""Keycloak authentication models."""
+
+from dataclasses import dataclass, field
+from typing import Any
+
+
+@dataclass(frozen=True)
+class TokenInfo:
+    """Information about validated token from Keycloak."""
+
+    active: bool
+    user_id: str = ""
+    username: str = ""
+    email: str = ""
+    roles: list[str] = field(default_factory=list)
+    raw_claims: dict[str, Any] = field(default_factory=dict, repr=False)
+
+    @property
+    def is_valid(self) -> bool:
+        """Check if token is valid and active."""
+        return self.active and bool(self.user_id)
+
+
+@dataclass(frozen=True)
+class KeycloakUser:
+    """User information from Keycloak."""
+
+    id: str
+    username: str
+    email: str
+    first_name: str = ""
+    last_name: str = ""
+    roles: list[str] = field(default_factory=list)
+    is_active: bool = True