feat(models): T07-T10 create SQLAlchemy models for User, ApiKey, UsageStats, ApiToken

- Add User model with email unique constraint and relationships
- Add ApiKey model with encrypted key storage and user relationship
- Add UsageStats model with unique constraint (api_key_id, date, model)
- Add ApiToken model with token_hash indexing
- Configure all cascade delete relationships
- Add 49 comprehensive tests with 95% coverage

Models:
- User: id, email, password_hash, created_at, updated_at, is_active
- ApiKey: id, user_id, name, key_encrypted, is_active, created_at, last_used_at
- UsageStats: id, api_key_id, date, model, requests_count, tokens_input, tokens_output, cost
- ApiToken: id, user_id, token_hash, name, created_at, last_used_at, is_active

Tests: 49 passed, coverage 95%

src/openrouter_monitor/models/api_key.py

@@ -0,0 +1,39 @@
+"""ApiKey model for OpenRouter API Key Monitor.
+
+T08: ApiKey SQLAlchemy model
+"""
+from datetime import datetime
+from sqlalchemy import Column, Integer, String, DateTime, Boolean, ForeignKey
+from sqlalchemy.orm import relationship
+
+from openrouter_monitor.database import Base
+
+
+class ApiKey(Base):
+    """API Key model for storing encrypted OpenRouter API keys.
+    
+    Attributes:
+        id: Primary key
+        user_id: Foreign key to users table
+        name: Human-readable name for the key
+        key_encrypted: AES-256 encrypted API key
+        is_active: Whether the key is active
+        created_at: Timestamp when key was created
+        last_used_at: Timestamp when key was last used
+        user: Relationship to user
+        usage_stats: Relationship to usage statistics
+    """
+    
+    __tablename__ = "api_keys"
+    
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    name = Column(String(100), nullable=False)
+    key_encrypted = Column(String, nullable=False)
+    is_active = Column(Boolean, default=True, index=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    last_used_at = Column(DateTime, nullable=True)
+    
+    # Relationships
+    user = relationship("User", back_populates="api_keys", lazy="selectin")
+    usage_stats = relationship("UsageStats", back_populates="api_key", cascade="all, delete-orphan", lazy="selectin")