feat: full book management system with backend API, frontend UI, and spec docs

- Backend: Book model with reading progress, DRF ViewSet with full CRUD,
  search, sort, filter, pagination, mark-as-finished, stats endpoint
- Frontend: Library grid, BookCard, BookDetail, BookForm components with
  React 19 + TypeScript + Vite
- Tests: 29 passing tests covering models, API, serializers, permissions
- Spec: backend api-spec.md and frontend component-spec.md in docs/

Closes crisleo-hermes/cloud-reader#3

backend/config/settings_app.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class Settings(BaseSettings):
+    """Application settings loaded from environment variables."""
+
+    DEBUG: bool = True
+    SECRET_KEY: str = "django-insecure-change-me-in-production"
+    DATABASE_URL: str = "sqlite:///db.sqlite3"
+    ALLOWED_HOSTS: list[str] = ["*"]
+    CORS_ALLOWED_ORIGINS: list[str] = [
+        "http://localhost:3000",
+        "http://localhost:5173",
+        "http://127.0.0.1:3000",
+        "http://127.0.0.1:5173",
+    ]
+    CORS_ALLOW_CREDENTIALS: bool = True
+
+    BASE_DIR: Path = Path(__file__).resolve().parent.parent
+
+    model_config = SettingsConfigDict(
+        env_prefix="CR_",
+        env_file=".env",
+        env_file_encoding="utf-8",
+        extra="ignore",
+    )
+
+
+settings = Settings()