feat(setup): T04 setup configuration files

- Create config.py with Pydantic Settings (SettingsConfigDict v2)
- Add all required configuration fields with defaults
- Create .env.example template with all environment variables
- Implement get_settings() with @lru_cache for performance
- Add test_configuration.py with 13 unit tests
- All tests passing (13/13)

Refs: T04

src/openrouter_monitor/config.py

@@ -0,0 +1,103 @@
+"""Configuration management using Pydantic Settings.
+
+This module provides centralized configuration management for the
+OpenRouter API Key Monitor application.
+"""
+from functools import lru_cache
+from pydantic_settings import BaseSettings, SettingsConfigDict
+from pydantic import Field
+
+
+class Settings(BaseSettings):
+    """Application settings loaded from environment variables.
+    
+    Required environment variables:
+        - SECRET_KEY: JWT signing key (min 32 chars)
+        - ENCRYPTION_KEY: AES-256 encryption key (32 bytes)
+    
+    Optional environment variables with defaults:
+        - DATABASE_URL: SQLite database path
+        - OPENROUTER_API_URL: OpenRouter API base URL
+        - SYNC_INTERVAL_MINUTES: Background sync interval
+        - MAX_API_KEYS_PER_USER: API key limit per user
+        - RATE_LIMIT_REQUESTS: API rate limit
+        - RATE_LIMIT_WINDOW: Rate limit window (seconds)
+        - JWT_EXPIRATION_HOURS: JWT token lifetime
+        - DEBUG: Debug mode flag
+        - LOG_LEVEL: Logging level
+    """
+    
+    # Database
+    database_url: str = Field(
+        default="sqlite:///./data/app.db",
+        description="SQLite database URL"
+    )
+    
+    # Security - REQUIRED
+    secret_key: str = Field(
+        description="JWT signing key (min 32 characters)"
+    )
+    encryption_key: str = Field(
+        description="AES-256 encryption key (32 bytes)"
+    )
+    jwt_expiration_hours: int = Field(
+        default=24,
+        description="JWT token expiration in hours"
+    )
+    
+    # OpenRouter Integration
+    openrouter_api_url: str = Field(
+        default="https://openrouter.ai/api/v1",
+        description="OpenRouter API base URL"
+    )
+    
+    # Task scheduling
+    sync_interval_minutes: int = Field(
+        default=60,
+        description="Background sync interval in minutes"
+    )
+    
+    # Limits
+    max_api_keys_per_user: int = Field(
+        default=10,
+        description="Maximum API keys per user"
+    )
+    rate_limit_requests: int = Field(
+        default=100,
+        description="API rate limit requests"
+    )
+    rate_limit_window: int = Field(
+        default=3600,
+        description="Rate limit window in seconds"
+    )
+    
+    # App settings
+    debug: bool = Field(
+        default=False,
+        description="Debug mode"
+    )
+    log_level: str = Field(
+        default="INFO",
+        description="Logging level"
+    )
+
+    model_config = SettingsConfigDict(
+        env_file=".env",
+        env_file_encoding="utf-8",
+        case_sensitive=False
+    )
+
+
+@lru_cache()
+def get_settings() -> Settings:
+    """Get cached settings instance.
+    
+    Returns:
+        Settings: Application settings instance
+        
+    Example:
+        >>> from openrouter_monitor.config import get_settings
+        >>> settings = get_settings()
+        >>> print(settings.database_url)
+    """
+    return Settings()