fix: network diagram team_id guard + multi-style edge routing

Backend:
- Guard create_diagram with 422 if current_user.team_id is None (prevents
  NOT NULL constraint crash for accounts not yet assigned to a team)
- Add routing field to DiagramEdge schema (straight/curved/step)

Frontend:
- ConnectionEdge now supports straight (default), curved (bezier), and
  step (smooth-step) routing per-edge via routing field in edge data
- PropertiesPanel Connection section gets a Line Style toggle:
  Straight | Curved | Step buttons, active state highlights in accent
- handleEdgeUpdate and serializeEdges now propagate the routing field
- DiagramEdge type gets optional routing field

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/network/panels/PropertiesPanel.tsx

@@ -1,5 +1,5 @@
 import { useCallback } from 'react'
-import { Trash2 } from 'lucide-react'
+import { Trash2, Minus, Spline, GitBranch } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import type { DeviceProperties, DiagramEdge } from '@/types'
 import type { Node, Edge } from '@xyflow/react'
@@ -139,6 +139,35 @@ export function PropertiesPanel({
               mono
             />
           </div>
+          <div className="flex flex-col gap-1">
+            <FieldLabel>Line Style</FieldLabel>
+            <div className="flex gap-1">
+              {([
+                { value: null, icon: Minus, label: 'Straight' },
+                { value: 'curved', icon: Spline, label: 'Curved' },
+                { value: 'step', icon: GitBranch, label: 'Step' },
+              ] as const).map(({ value, icon: Icon, label }) => {
+                const routing = (edgeData.routing as string | null | undefined) ?? null
+                const active = routing === value
+                return (
+                  <button
+                    key={label}
+                    title={label}
+                    onClick={() => onEdgeUpdate(selectedEdge.id, { routing: value })}
+                    className={cn(
+                      'flex flex-1 items-center justify-center gap-1 rounded border py-1.5 text-[10px] transition-colors',
+                      active
+                        ? 'border-accent bg-accent/10 text-accent'
+                        : 'border-default text-muted-foreground hover:border-hover hover:text-primary',
+                    )}
+                  >
+                    <Icon size={12} />
+                    {label}
+                  </button>
+                )
+              })}
+            </div>
+          </div>
           <div className="flex items-center justify-between">
             <FieldLabel>Show Traffic</FieldLabel>
             <button