STATE-WP-0069 T04/T06: body metering and workplan-first state internals

Share LegacyWorkstreamIdBodyMixin across create schemas; meter POST /tasks/
and /decisions/ workstream_id bodies. State summary uses workplan flow;
NextStep dual-writes workplan_* fields alongside legacy workstream_*.

api/schemas/compat.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 import uuid
+from typing import Any
 
 from pydantic import AliasChoices, Field, computed_field, model_validator
 
@@ -13,6 +14,20 @@ def workplan_id_field(*, default: uuid.UUID | None = None) -> uuid.UUID | None:
     )
 
 
+class LegacyWorkstreamIdBodyMixin:
+    """Detect POST/PUT bodies that used legacy ``workstream_id`` without ``workplan_id``."""
+
+    used_legacy_workstream_id: bool = Field(default=False, exclude=True)
+
+    @model_validator(mode="before")
+    @classmethod
+    def _detect_legacy_workstream_id(cls, data: Any) -> Any:
+        if isinstance(data, dict):
+            if data.get("workstream_id") is not None and data.get("workplan_id") is None:
+                return {**data, "used_legacy_workstream_id": True}
+        return data
+
+
 class WorkplanIdCompatMixin:
     """Accept ``workplan_id`` or legacy ``workstream_id`` on input; emit both on output."""
 
@@ -24,7 +39,7 @@ class WorkplanIdCompatMixin:
         return self.workplan_id
 
 
-class WorkplanIdCreateMixin:
+class WorkplanIdCreateMixin(LegacyWorkstreamIdBodyMixin):
     workplan_id: uuid.UUID | None = workplan_id_field(default=None)
 
     @model_validator(mode="after")