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

.env.example

@@ -0,0 +1,29 @@
+# ===========================================
+# OpenRouter API Key Monitor - Configuration
+# ===========================================
+
+# Database
+DATABASE_URL=sqlite:///./data/app.db
+
+# Security - REQUIRED
+# Generate with: openssl rand -hex 32
+SECRET_KEY=your-super-secret-jwt-key-min-32-chars
+ENCRYPTION_KEY=your-32-byte-encryption-key-here
+
+# OpenRouter Integration
+OPENROUTER_API_URL=https://openrouter.ai/api/v1
+
+# Background Tasks
+SYNC_INTERVAL_MINUTES=60
+
+# Limits
+MAX_API_KEYS_PER_USER=10
+RATE_LIMIT_REQUESTS=100
+RATE_LIMIT_WINDOW=3600
+
+# JWT
+JWT_EXPIRATION_HOURS=24
+
+# Development
+DEBUG=false
+LOG_LEVEL=INFO