fix: backend code review fixes for network diagrams

- Replace legacy Optional imports with modern str | None syntax
- Type JSONB columns as Mapped[list[dict[str, Any]]]
- Escape SQL LIKE wildcards (%, _) in diagram search
- Type DiagramNode.position as Position(x, y) Pydantic model
- Wrap AI response parsing in KeyError handler for clean 422 errors
- Remove unused Optional/TYPE_CHECKING imports from schemas/models
- Extract _get_available_slugs helper to DRY duplicate queries

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/network_diagrams.py

@@ -62,6 +62,14 @@ def _diagram_to_list_item(diagram: NetworkDiagram) -> NetworkDiagramListItem:
     )
 
 
+async def _get_available_slugs(team_id: UUID, db: AsyncSession) -> set[str]:
+    stmt = select(DeviceType.slug).where(
+        or_(DeviceType.is_system.is_(True), DeviceType.team_id == team_id)
+    )
+    result = await db.execute(stmt)
+    return {row[0] for row in result.all()}
+
+
 @router.get("/clients", response_model=list[str])
 async def list_client_names(
     db: Annotated[AsyncSession, Depends(get_db)],
@@ -102,7 +110,8 @@ async def list_diagrams(
         stmt = stmt.where(NetworkDiagram.client_name == client_name)
 
     if search:
-        search_filter = f"%{search}%"
+        escaped = search.replace("\\", "\\\\").replace("%", "\\%").replace("_", "\\_")
+        search_filter = f"%{escaped}%"
         stmt = stmt.where(
             or_(
                 NetworkDiagram.name.ilike(search_filter),
@@ -232,14 +241,7 @@ async def import_diagram(
     db: Annotated[AsyncSession, Depends(get_db)],
     current_user: Annotated[User, Depends(get_current_active_user)],
 ) -> DiagramImportResponse:
-    available_stmt = select(DeviceType.slug).where(
-        or_(
-            DeviceType.is_system.is_(True),
-            DeviceType.team_id == current_user.team_id,
-        )
-    )
-    result = await db.execute(available_stmt)
-    available_slugs = {row[0] for row in result.all()}
+    available_slugs = await _get_available_slugs(current_user.team_id, db)
 
     warnings: list[str] = []
     for node in data.nodes:
@@ -271,14 +273,8 @@ async def ai_generate_diagram(
     db: Annotated[AsyncSession, Depends(get_db)],
     current_user: Annotated[User, Depends(get_current_active_user)],
 ) -> AIGenerateResponse:
-    stmt = select(DeviceType.slug).where(
-        or_(
-            DeviceType.is_system.is_(True),
-            DeviceType.team_id == current_user.team_id,
-        )
-    )
-    result = await db.execute(stmt)
-    available_slugs = [row[0] for row in result.all()]
+    available_slugs_set = await _get_available_slugs(current_user.team_id, db)
+    available_slugs = list(available_slugs_set)
 
     existing_node_ids: list[str] | None = None
     if data.mode == "merge" and data.existingBounds: