feat: arrowhead endpoints for edges + fix parallel edges not rendering

Add optional filled-triangle arrowheads at either end of an edge,
independently toggleable per edge (EdgeModal) and as per-edge-type
defaults (CustomStyleModal). Arrowheads are custom inline <marker> defs
filled with the live stroke colour so they recolour reactively with
custom_color / vlan / selected state. Persisted frontend (serializer)
and backend (edge columns + schemas + runtime migration).

Also fix two dedupe layers that silently dropped legitimate parallel
links between the same two devices:
- store: React Flow addEdge() connectionExists dropped a second edge
  with matching source+target when handles were null/equal. Build the
  edge with a unique id and append directly.
- render: rewireEdgesForCollapse deduped ALL edges by src->tgt key even
  when nothing was collapsed, filtering real parallel edges out of the
  visible set. Restrict the anti-mesh dedupe to rewired collapse stubs.

Tests: marker render, per-edge/per-type UI, store apply, serializer
round-trip, backend edge/canvas persistence, parallel-edge regressions.

ha-relevant: yes

frontend/src/components/canvas/edges/index.tsx

@@ -351,9 +351,48 @@ export function HomelableEdge({ id, source, target, sourceHandleId, targetHandle
     ? segmentMidpoints(sourceX, sourceY, waypoints, targetX, targetY, pathStyle, sourcePosition)
     : []
 
+  // ── Arrowheads ─────────────────────────────────────────────────────────────
+  // Custom inline <marker> defs filled with the live strokeColor so they recolor
+  // reactively (custom_color / vlan / selected). Sized from the stroke width.
+  const markerStart = data?.marker_start === true
+  const markerEnd = data?.marker_end === true
+  const strokeW = (style.strokeWidth as number) ?? 2
+  const markerSize = 6 + strokeW * 2
+  const startMarkerId = `arrow-start-${id}`
+  const endMarkerId = `arrow-end-${id}`
+
+  const arrowMarker = (markerId: string, orient: string) => (
+    <marker
+      id={markerId}
+      viewBox="0 0 10 10"
+      refX={9}
+      refY={5}
+      markerWidth={markerSize}
+      markerHeight={markerSize}
+      markerUnits="userSpaceOnUse"
+      orient={orient}
+    >
+      <path d="M 0 0 L 10 5 L 0 10 z" fill={strokeColor} />
+    </marker>
+  )
+
   return (
     <>
-      <BaseEdge id={id} path={edgePath} style={animMode === 'basic' ? { ...style, stroke: 'transparent' } : style} interactionWidth={16} />
+      {(markerStart || markerEnd) && (
+        <defs>
+          {markerStart && arrowMarker(startMarkerId, 'auto-start-reverse')}
+          {markerEnd && arrowMarker(endMarkerId, 'auto')}
+        </defs>
+      )}
+
+      <BaseEdge
+        id={id}
+        path={edgePath}
+        style={animMode === 'basic' ? { ...style, stroke: 'transparent' } : style}
+        interactionWidth={16}
+        markerStart={markerStart ? `url(#${startMarkerId})` : undefined}
+        markerEnd={markerEnd ? `url(#${endMarkerId})` : undefined}
+      />
 
       {animMode === 'basic' && (
         <path