feat: flow engine data layer with tests

- SQLModel tables: flow_definitions/versions/runs, approval_tasks
- Alembic setup with autogenerate (sqlmodel types fixed via script.py.mako)
- Settings extended (shared root .env), db engine/session helpers
- pytest suite: conftest with isolated test DB, 5 model tests green

backend/migrations/versions/62ea9a5d8f9d_flow_engine_tables.py

@@ -0,0 +1,108 @@
+"""flow engine tables
+
+Revision ID: 62ea9a5d8f9d
+Revises: 
+Create Date: 2026-09-06 22:27:07.129788
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+import sqlmodel
+
+
+# revision identifiers, used by Alembic.
+revision: str = '62ea9a5d8f9d'
+down_revision: Union[str, Sequence[str], None] = None
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('flow_definitions',
+    sa.Column('id', sa.Uuid(), nullable=False),
+    sa.Column('name', sqlmodel.sql.sqltypes.AutoString(length=200), nullable=False),
+    sa.Column('description', sqlmodel.sql.sqltypes.AutoString(length=2000), nullable=False),
+    sa.Column('status', sqlmodel.sql.sqltypes.AutoString(length=20), nullable=False),
+    sa.Column('continuation_flow_id', sa.Uuid(), nullable=True),
+    sa.Column('created_at', sa.DateTime(timezone=True), nullable=True),
+    sa.ForeignKeyConstraint(['continuation_flow_id'], ['flow_definitions.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_flow_definitions_name'), 'flow_definitions', ['name'], unique=True)
+    op.create_table('flow_versions',
+    sa.Column('id', sa.Uuid(), nullable=False),
+    sa.Column('flow_id', sa.Uuid(), nullable=False),
+    sa.Column('version_no', sa.Integer(), nullable=False),
+    sa.Column('langflow_json', sa.JSON(), nullable=True),
+    sa.Column('lfx_flow_id', sqlmodel.sql.sqltypes.AutoString(length=200), nullable=True),
+    sa.Column('deployed_at', sa.DateTime(timezone=True), nullable=True),
+    sa.Column('author', sqlmodel.sql.sqltypes.AutoString(length=200), nullable=False),
+    sa.Column('created_at', sa.DateTime(timezone=True), nullable=True),
+    sa.ForeignKeyConstraint(['flow_id'], ['flow_definitions.id'], ),
+    sa.PrimaryKeyConstraint('id'),
+    sa.UniqueConstraint('flow_id', 'version_no')
+    )
+    op.create_index(op.f('ix_flow_versions_flow_id'), 'flow_versions', ['flow_id'], unique=False)
+    op.create_index(op.f('ix_flow_versions_version_no'), 'flow_versions', ['version_no'], unique=False)
+    op.create_table('flow_runs',
+    sa.Column('id', sa.Uuid(), nullable=False),
+    sa.Column('version_id', sa.Uuid(), nullable=False),
+    sa.Column('tenant_id', sqlmodel.sql.sqltypes.AutoString(length=200), nullable=False),
+    sa.Column('status', sqlmodel.sql.sqltypes.AutoString(length=30), nullable=False),
+    sa.Column('input', sa.JSON(), nullable=True),
+    sa.Column('output', sa.JSON(), nullable=True),
+    sa.Column('session_id', sqlmodel.sql.sqltypes.AutoString(length=200), nullable=True),
+    sa.Column('created_at', sa.DateTime(timezone=True), nullable=True),
+    sa.Column('finished_at', sa.DateTime(timezone=True), nullable=True),
+    sa.ForeignKeyConstraint(['version_id'], ['flow_versions.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_flow_runs_status'), 'flow_runs', ['status'], unique=False)
+    op.create_index(op.f('ix_flow_runs_tenant_id'), 'flow_runs', ['tenant_id'], unique=False)
+    op.create_index(op.f('ix_flow_runs_version_id'), 'flow_runs', ['version_id'], unique=False)
+    op.create_table('approval_tasks',
+    sa.Column('id', sa.Uuid(), nullable=False),
+    sa.Column('run_id', sa.Uuid(), nullable=False),
+    sa.Column('version_id', sa.Uuid(), nullable=False),
+    sa.Column('tenant_id', sqlmodel.sql.sqltypes.AutoString(length=200), nullable=False),
+    sa.Column('title', sqlmodel.sql.sqltypes.AutoString(length=500), nullable=False),
+    sa.Column('payload', sa.JSON(), nullable=True),
+    sa.Column('continuation_flow_id', sa.Uuid(), nullable=True),
+    sa.Column('approver', sqlmodel.sql.sqltypes.AutoString(length=200), nullable=True),
+    sa.Column('status', sqlmodel.sql.sqltypes.AutoString(length=20), nullable=False),
+    sa.Column('decision_note', sqlmodel.sql.sqltypes.AutoString(length=2000), nullable=False),
+    sa.Column('created_at', sa.DateTime(timezone=True), nullable=True),
+    sa.Column('decided_at', sa.DateTime(timezone=True), nullable=True),
+    sa.ForeignKeyConstraint(['run_id'], ['flow_runs.id'], ),
+    sa.ForeignKeyConstraint(['version_id'], ['flow_versions.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_approval_tasks_run_id'), 'approval_tasks', ['run_id'], unique=False)
+    op.create_index(op.f('ix_approval_tasks_status'), 'approval_tasks', ['status'], unique=False)
+    op.create_index(op.f('ix_approval_tasks_tenant_id'), 'approval_tasks', ['tenant_id'], unique=False)
+    op.create_index(op.f('ix_approval_tasks_version_id'), 'approval_tasks', ['version_id'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_approval_tasks_version_id'), table_name='approval_tasks')
+    op.drop_index(op.f('ix_approval_tasks_tenant_id'), table_name='approval_tasks')
+    op.drop_index(op.f('ix_approval_tasks_status'), table_name='approval_tasks')
+    op.drop_index(op.f('ix_approval_tasks_run_id'), table_name='approval_tasks')
+    op.drop_table('approval_tasks')
+    op.drop_index(op.f('ix_flow_runs_version_id'), table_name='flow_runs')
+    op.drop_index(op.f('ix_flow_runs_tenant_id'), table_name='flow_runs')
+    op.drop_index(op.f('ix_flow_runs_status'), table_name='flow_runs')
+    op.drop_table('flow_runs')
+    op.drop_index(op.f('ix_flow_versions_version_no'), table_name='flow_versions')
+    op.drop_index(op.f('ix_flow_versions_flow_id'), table_name='flow_versions')
+    op.drop_table('flow_versions')
+    op.drop_index(op.f('ix_flow_definitions_name'), table_name='flow_definitions')
+    op.drop_table('flow_definitions')
+    # ### end Alembic commands ###