feat: add app foundation (config.py, app.py, home page)

- config.py: Pydantic BaseSettings for env loading
- app.py: Dash factory with Pages routing
- pages/home.py: Placeholder landing page

Closes #7

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

portfolio_app/config.py

@@ -0,0 +1,34 @@
+"""Application configuration using Pydantic BaseSettings."""
+
+from functools import lru_cache
+
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class Settings(BaseSettings):
+    """Application settings loaded from environment variables."""
+
+    model_config = SettingsConfigDict(
+        env_file=".env",
+        env_file_encoding="utf-8",
+        extra="ignore",
+    )
+
+    # Database
+    database_url: str = "postgresql://portfolio:portfolio_dev@localhost:5432/portfolio"
+    postgres_user: str = "portfolio"
+    postgres_password: str = "portfolio_dev"
+    postgres_db: str = "portfolio"
+
+    # Application
+    dash_debug: bool = True
+    secret_key: str = "change-me-in-production"
+
+    # Logging
+    log_level: str = "INFO"
+
+
+@lru_cache
+def get_settings() -> Settings:
+    """Get cached settings instance."""
+    return Settings()