feat: add batch_id/target_label to sessions and batch launch endpoint

- Add batch_id (UUID, nullable, indexed) and target_label (String 255,
  nullable) columns to the Session model
- Manual Alembic migration 6e8128ef2aa8 applies both columns
- POST /sessions/batch creates one session per target for maintenance
  flows; rejects empty targets (422) and non-maintenance trees (400)
- SessionResponse schema exposes batch_id and target_label
- 3 new integration tests, all 540 tests pass

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/api/endpoints/sessions.py

@@ -485,3 +485,83 @@ async def save_session_as_tree(
         tree_name=new_tree.name,
         message=f"Session saved as {'published' if request_data.status == 'published' else 'draft'} tree"
     )
+
+
+# ── Batch Launch (Maintenance Flows) ──────────────────────────────────────
+
+from pydantic import BaseModel, Field as PydanticField
+import uuid as uuid_mod
+
+
+class _BatchTarget(BaseModel):
+    label: str = PydanticField(..., min_length=1, max_length=255)
+    notes: Optional[str] = None
+
+
+class _BatchLaunchRequest(BaseModel):
+    tree_id: UUID
+    targets: list[_BatchTarget] = PydanticField(..., min_length=1)
+
+
+class _BatchLaunchResponse(BaseModel):
+    batch_id: str
+    count: int
+    sessions: list[dict]
+
+
+@router.post("/batch", status_code=201, response_model=_BatchLaunchResponse)
+async def batch_launch_sessions(
+    data: _BatchLaunchRequest,
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+):
+    """Create one session per target for a maintenance flow batch run."""
+    tree_result = await db.execute(select(Tree).where(Tree.id == data.tree_id))
+    tree = tree_result.scalar_one_or_none()
+    if not tree:
+        raise HTTPException(status_code=404, detail="Tree not found")
+    if tree.tree_type != "maintenance":
+        raise HTTPException(status_code=400, detail="Batch launch is only for maintenance flows")
+
+    batch_id = uuid_mod.uuid4()
+    created_sessions = []
+
+    for target in data.targets:
+        tree_snapshot = {
+            **tree.tree_structure,
+            "name": tree.name,
+            "description": tree.description,
+            "tree_type": tree.tree_type,
+        }
+        session = Session(
+            tree_id=tree.id,
+            user_id=current_user.id,
+            tree_snapshot=tree_snapshot,
+            path_taken=[],
+            decisions=[],
+            custom_steps=[],
+            session_variables={},
+            batch_id=batch_id,
+            target_label=target.label,
+        )
+        db.add(session)
+        created_sessions.append(session)
+
+    await db.flush()
+    for s in created_sessions:
+        await db.refresh(s)
+    await db.commit()
+
+    return _BatchLaunchResponse(
+        batch_id=str(batch_id),
+        count=len(created_sessions),
+        sessions=[
+            {
+                "id": str(s.id),
+                "batch_id": str(s.batch_id),
+                "target_label": s.target_label,
+                "tree_id": str(s.tree_id),
+            }
+            for s in created_sessions
+        ],
+    )