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/task.py

@@ -0,0 +1,41 @@
+"""
+Task model for SQLAlchemy ORM
+"""
+
+from sqlalchemy import Column, String, Text, Integer, 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 Task(Base):
+    __tablename__ = "tasks"
+    
+    # Primary identification
+    id = Column(SqlUUID(as_uuid=True), primary_key=True, index=True, default=uuid.uuid4)
+    
+    # Task details
+    title = Column(String(255), nullable=False)
+    description = Column(Text)
+    priority = Column(Integer, default=5)
+    status = Column(String(50), default='pending')
+    
+    # Relationships
+    assigned_agent_id = Column(String(255), ForeignKey("agents.id"), nullable=True)
+    workflow_id = Column(SqlUUID(as_uuid=True), ForeignKey("workflows.id"), nullable=True)
+    execution_id = Column(SqlUUID(as_uuid=True), ForeignKey("executions.id"), nullable=True)
+    
+    # Metadata and context
+    metadata = Column(JSONB, nullable=True)
+    
+    # Timestamps
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    started_at = Column(DateTime(timezone=True), nullable=True)
+    completed_at = Column(DateTime(timezone=True), nullable=True)
+    
+    # Relationships
+    assigned_agent = relationship("Agent", back_populates="tasks")
+    workflow = relationship("Workflow", back_populates="tasks")
+    execution = relationship("Execution", back_populates="tasks")