feat(workspaces): return 409 name_conflict instead of 500 on Library name clash

A recreate of a workspace whose Mnemosyne Library was orphaned (left behind
by a failed Daedalus delete-propagate) collides on the global Library.name
unique constraint. neomodel raised UniqueProperty unguarded, so workspace_create
500'd and ingest then 404'd forever — the queue froze silently.

Guard lib.save() and return a structured 409 with a machine code so Daedalus
can classify the failure without string-matching:
- name_conflict   — the new name-collision case
- owner_conflict, library_type_immutable — codes added to the two existing 409s

Cypher-touching paths stay covered by the manual end-to-end plan, per the
test module's stated convention.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mnemosyne/library/api/workspaces.py

@@ -17,6 +17,7 @@ across users.
 import logging
 
 from neomodel import db
+from neomodel.exceptions import UniqueProperty
 from rest_framework import status
 from rest_framework.decorators import api_view, permission_classes
 from rest_framework.permissions import IsAuthenticated
@@ -85,7 +86,10 @@ def workspace_create(request):
                 data["workspace_id"], request.user.username,
             )
             return Response(
-                {"detail": "Workspace id is already in use."},
+                {
+                    "detail": "Workspace id is already in use.",
+                    "code": "owner_conflict",
+                },
                 status=status.HTTP_409_CONFLICT,
             )
         if existing.library_type != data["library_type"]:
@@ -95,7 +99,8 @@ def workspace_create(request):
                         "library_type is immutable for an existing workspace "
                         f"(have '{existing.library_type}', "
                         f"got '{data['library_type']}')."
-                    )
+                    ),
+                    "code": "library_type_immutable",
                 },
                 status=status.HTTP_409_CONFLICT,
             )
@@ -120,7 +125,29 @@ def workspace_create(request):
         reranker_instruction=defaults["reranker_instruction"],
         llm_context_prompt=defaults["llm_context_prompt"],
     )
+    try:
         lib.save()
+    except UniqueProperty:
+        # Library.name is globally unique. A name collision here almost always
+        # means an orphaned Library survived a failed Daedalus workspace delete
+        # (the old node kept the name), and the recreate under a new
+        # workspace_id now clashes. Surface a clean 409 instead of a 500 so
+        # Daedalus can record + report it; the operator clears the orphan
+        # (admin delete) or renames the workspace.
+        logger.warning(
+            "workspace_create name_conflict workspace_id=%s name=%s",
+            data["workspace_id"], data["name"],
+        )
+        return Response(
+            {
+                "detail": (
+                    f"A library named '{data['name']}' already exists in "
+                    "Mnemosyne."
+                ),
+                "code": "name_conflict",
+            },
+            status=status.HTTP_409_CONFLICT,
+        )
     logger.info(
         "Workspace created workspace_id=%s library_uid=%s library_type=%s",
         data["workspace_id"], lib.uid, lib.library_type,