feat: replace SSE streaming with job-based LangFlow V2 workflow

[cfchase]

Summary

• Replace SSE streaming with LangFlow V2 Workflow API's job-based async execution model
• Add Job model and API endpoints for tracking long-running LangFlow executions
• Frontend polls job status instead of consuming SSE streams
• Upgrade LangFlow to 1.8.0 with V2 API support
• Add integration tests for the full job lifecycle

Test plan

• Run make test-backend to verify all backend tests pass
• Run make test-frontend to verify frontend tests pass
• Manual test: send a chat message and verify job polling completes
• Verify job cancellation works via the stop button

🤖 Generated with Claude Code

[cfchase]

Code review

Found 1 issue:

1. Missing ondelete="CASCADE" on the Job.chat_message_id foreign key. The backend/CLAUDE.md documents cascade rules: "Join Tables: Use ondelete="CASCADE" on foreign keys" and "One-to-Many: CASCADE to owned children". Every other foreign key in the codebase follows this pattern (ChatMessage.chat_id, Chat.user_id, Item.owner_id, UserIntegration.user_id), but Job.chat_message_id omits it. Without CASCADE, deleting a ChatMessage will either raise a foreign key constraint violation or leave orphaned Job records. The migration file also needs the corresponding ondelete='CASCADE' on its ForeignKeyConstraint.

multi-agent-platform/backend/app/models/job.py

Lines 44 to 47 in 380a649

	id: int | None = Field(default=None, primary_key=True)
	chat_message_id: int = Field(
	foreign_key="chat_message.id", nullable=False, index=True
	)

multi-agent-platform/backend/app/alembic/versions/a1b2c3d4e5f6_add_job_table.py

Lines 32 to 34 in 380a649

	sa.Column('updated_at', sa.DateTime(timezone=True), nullable=False),
	sa.ForeignKeyConstraint(['chat_message_id'], ['chat_message.id'], ),
	sa.PrimaryKeyConstraint('id')

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[cfchase]

Closed: changes were squash merged directly to main in db8b39a.