Fix critical in-memory task storage with database persistence

Major architectural improvement to replace in-memory task storage with
database-backed persistence while maintaining backward compatibility.

Changes:
- Created Task SQLAlchemy model matching database schema
- Added Workflow and Execution SQLAlchemy models
- Created TaskService for database CRUD operations
- Updated UnifiedCoordinator to use database persistence
- Modified task APIs to leverage database storage
- Added task loading from database on coordinator initialization
- Implemented status change persistence during task execution
- Enhanced task cleanup with database support
- Added comprehensive task statistics from database

Benefits:
- Tasks persist across application restarts
- Better scalability and reliability
- Historical task data retention
- Comprehensive task filtering and querying
- Maintains in-memory cache for performance

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/app/models/user.py

@@ -44,6 +44,7 @@ class User(Base):
     # Relationships for authentication features
     api_keys = relationship("APIKey", back_populates="user", cascade="all, delete-orphan")
     refresh_tokens = relationship("RefreshToken", back_populates="user", cascade="all, delete-orphan")
+    workflows = relationship("Workflow", back_populates="creator")
     
     def verify_password(self, password: str) -> bool:
         """Verify a password against the hashed password."""