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/sqlalchemy_models.py

@@ -0,0 +1,63 @@
+"""
+SQLAlchemy models for workflows and executions
+"""
+
+from sqlalchemy import Column, String, Text, Integer, Boolean, DateTime, ForeignKey, UUID as SqlUUID
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from ..core.database import Base
+import uuid
+
+
+class Workflow(Base):
+    __tablename__ = "workflows"
+    
+    # Primary identification
+    id = Column(SqlUUID(as_uuid=True), primary_key=True, index=True, default=uuid.uuid4)
+    
+    # Workflow details
+    name = Column(String(255), nullable=False)
+    description = Column(Text)
+    n8n_data = Column(JSONB, nullable=False)
+    mcp_tools = Column(JSONB)
+    
+    # Relationships
+    created_by = Column(SqlUUID(as_uuid=True), ForeignKey("users.id"))
+    
+    # Metadata
+    version = Column(Integer, default=1)
+    active = Column(Boolean, default=True)
+    
+    # Timestamps
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+    
+    # Relationships
+    creator = relationship("User", back_populates="workflows")
+    executions = relationship("Execution", back_populates="workflow")
+    tasks = relationship("Task", back_populates="workflow")
+
+
+class Execution(Base):
+    __tablename__ = "executions"
+    
+    # Primary identification
+    id = Column(SqlUUID(as_uuid=True), primary_key=True, index=True, default=uuid.uuid4)
+    
+    # Execution details
+    workflow_id = Column(SqlUUID(as_uuid=True), ForeignKey("workflows.id"), nullable=True)
+    status = Column(String(50), default='pending')
+    input_data = Column(JSONB)
+    output_data = Column(JSONB)
+    error_message = Column(Text)
+    progress = Column(Integer, default=0)
+    
+    # Timestamps
+    started_at = Column(DateTime(timezone=True), nullable=True)
+    completed_at = Column(DateTime(timezone=True), nullable=True)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    
+    # Relationships
+    workflow = relationship("Workflow", back_populates="executions")
+    tasks = relationship("Task", back_populates="execution")