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/schemas/device_type.py

@@ -1,6 +1,5 @@
 """Pydantic schemas for device types."""
 from datetime import datetime
-from typing import Optional
 from uuid import UUID
 
 from pydantic import BaseModel, Field

backend/app/schemas/network_diagram.py

@@ -1,11 +1,15 @@
 """Pydantic schemas for network diagrams."""
 from datetime import datetime
-from typing import Optional
 from uuid import UUID
 
 from pydantic import BaseModel, Field
 
 
+class Position(BaseModel):
+    x: float
+    y: float
+
+
 class DeviceProperties(BaseModel):
     hostname: str | None = None
     ip: str | None = None
@@ -22,7 +26,7 @@ class DiagramNode(BaseModel):
     id: str
     type: str
     label: str
-    position: dict
+    position: Position
     properties: DeviceProperties = Field(default_factory=DeviceProperties)