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/app/models/__init__.py

@@ -0,0 +1,3 @@
+from .flow import ApprovalTask, FlowDefinition, FlowRun, FlowVersion
+
+__all__ = ["ApprovalTask", "FlowDefinition", "FlowRun", "FlowVersion"]

backend/app/models/flow.py

@@ -0,0 +1,90 @@
+"""Flow engine domain models (SQLModel tables)."""
+
+from datetime import datetime, timezone
+from uuid import UUID, uuid4
+
+from sqlalchemy import JSON, Column, DateTime, UniqueConstraint
+from sqlmodel import Field, SQLModel
+
+
+def utcnow() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+def _ts_column() -> Column:
+    return Column(DateTime(timezone=True))
+
+
+def _json_column() -> Column:
+    # Generic JSON: JSONB on Postgres, JSON/TEXT elsewhere.
+    return Column(JSON)
+
+
+class FlowDefinition(SQLModel, table=True):
+    """A named decision flow (e.g. 'Bauvorhaben pruefen'). Versions live separately."""
+
+    __tablename__ = "flow_definitions"
+
+    id: UUID = Field(default_factory=uuid4, primary_key=True)
+    name: str = Field(index=True, unique=True, max_length=200)
+    description: str = Field(default="", max_length=2000)
+    status: str = Field(default="active", max_length=20)  # active | archived
+    # Flow to run after a human approval on this flow's runs (nullable).
+    continuation_flow_id: UUID | None = Field(
+        default=None, foreign_key="flow_definitions.id"
+    )
+    created_at: datetime = Field(default_factory=utcnow, sa_column=_ts_column())
+
+
+class FlowVersion(SQLModel, table=True):
+    """An immutable version of a flow: the exact Langflow JSON that runs."""
+
+    __tablename__ = "flow_versions"
+    __table_args__ = (UniqueConstraint("flow_id", "version_no"),)
+
+    id: UUID = Field(default_factory=uuid4, primary_key=True)
+    flow_id: UUID = Field(foreign_key="flow_definitions.id", index=True)
+    version_no: int = Field(index=True)
+    langflow_json: dict = Field(default_factory=dict, sa_column=_json_column())
+    # Flow id under which LFX serves this version (set on deploy).
+    lfx_flow_id: str | None = Field(default=None, max_length=200)
+    deployed_at: datetime | None = Field(default=None, sa_column=_ts_column())
+    author: str = Field(default="", max_length=200)
+    created_at: datetime = Field(default_factory=utcnow, sa_column=_ts_column())
+
+
+class FlowRun(SQLModel, table=True):
+    """One execution of a flow version."""
+
+    __tablename__ = "flow_runs"
+
+    id: UUID = Field(default_factory=uuid4, primary_key=True)
+    version_id: UUID = Field(foreign_key="flow_versions.id", index=True)
+    tenant_id: str = Field(default="default", index=True, max_length=200)
+    # running | completed | pending_approval | failed
+    status: str = Field(default="running", index=True, max_length=30)
+    input: dict | None = Field(default=None, sa_column=_json_column())
+    output: dict | None = Field(default=None, sa_column=_json_column())
+    session_id: str | None = Field(default=None, max_length=200)
+    created_at: datetime = Field(default_factory=utcnow, sa_column=_ts_column())
+    finished_at: datetime | None = Field(default=None, sa_column=_ts_column())
+
+
+class ApprovalTask(SQLModel, table=True):
+    """Human task created by a flow that reached a terminal approval state."""
+
+    __tablename__ = "approval_tasks"
+
+    id: UUID = Field(default_factory=uuid4, primary_key=True)
+    run_id: UUID = Field(foreign_key="flow_runs.id", index=True)
+    version_id: UUID = Field(foreign_key="flow_versions.id", index=True)
+    tenant_id: str = Field(default="default", index=True, max_length=200)
+    title: str = Field(default="", max_length=500)
+    payload: dict = Field(default_factory=dict, sa_column=_json_column())
+    continuation_flow_id: UUID | None = Field(default=None)
+    approver: str | None = Field(default=None, max_length=200)
+    # open | approved | rejected
+    status: str = Field(default="open", index=True, max_length=20)
+    decision_note: str = Field(default="", max_length=2000)
+    created_at: datetime = Field(default_factory=utcnow, sa_column=_ts_column())
+    decided_at: datetime | None = Field(default=None, sa_column=_ts_column())