feat(investigations): add event impact and work-order feedback flows

Add isolated burst and pollution impact evidence, parallel work-order workflows, persisted notifications and graph updates.

Checkpoint current code. Known follow-ups remain for large pollution result previews and graceful geometry failure.

app/services/investigation_graph.py

@@ -23,10 +23,11 @@ def adopt_legacy_investigation_graph(diagram: InvestigationDiagram) -> Investiga
 def update_investigation_graph(
     previous: InvestigationDiagram | None, update: InvestigationGraphUpdate,
 ) -> InvestigationDiagram:
-    """Upsert nodes and append explicit edges; never delete a path or erase a failed attempt.
+    """Patch saved nodes or add labelled nodes and explicit edges; never erase a path.
 
-    A retry uses a new node id. Relationships describe activity for readers and
-    impose no ordering or completion requirements on the Agent.
+    Unspecified fields retain their saved values. A failed attempt remains
+    failed, and a retry uses a new node id. Relationships impose no execution
+    ordering or completion requirements on the Agent.
     """
     if previous is not None:
         previous = adopt_legacy_investigation_graph(previous)
@@ -47,15 +48,19 @@ def update_investigation_graph(
         if existing is None:
             require(not in_source(node.node_id), "GRAPH_LEGACY_NODE_CONFLICT",
                     "This id belongs to the original Mermaid; use a new node id", status=409)
+            require(node.label is not None, "GRAPH_NODE_LABEL_REQUIRED",
+                    "A new graph node requires a label; use an existing node_id for a partial update", status=422)
             nodes[node.node_id] = InvestigationDiagramNodeRef.model_validate(node.model_dump(exclude_unset=True))
             continue
-        require(not in_source(node.node_id) or existing.label == node.label,
+        require(not in_source(node.node_id) or node.label is None or existing.label == node.label,
                 "GRAPH_LEGACY_NODE_CONFLICT", "Keep the original node label; use a new node for a new step", status=409)
         require(existing.status != "failed" or (
             ("status" not in node.model_fields_set or node.status == "failed")
-            and existing.label == node.label
+            and (node.label is None or existing.label == node.label)
         ), "GRAPH_FAILED_ATTEMPT_IMMUTABLE", "Keep the failed attempt; use a new node for a retry", status=409)
         changes = node.model_dump(exclude_unset=True)
+        if node.label is None:
+            changes.pop("label", None)
         if existing.status == "failed" and existing.detail and not changes.get("detail"):
             changes.pop("detail", None)
         for field in ("evidence_refs", "action_refs", "map_targets"):