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/utils/__tests__/canvasSerializer.test.ts

@@ -243,6 +243,19 @@ describe('serializeEdge', () => {
     expect(result.animated).toBe(true)
   })
 
+  it('serializes arrow markers', () => {
+    const edge = makeRfEdge({ data: { type: 'ethernet', marker_start: true, marker_end: true } })
+    const result = serializeEdge(edge)
+    expect(result.marker_start).toBe(true)
+    expect(result.marker_end).toBe(true)
+  })
+
+  it('defaults arrow markers to false when absent', () => {
+    const result = serializeEdge(makeRfEdge())
+    expect(result.marker_start).toBe(false)
+    expect(result.marker_end).toBe(false)
+  })
+
   it('nulls optional fields when absent', () => {
     const result = serializeEdge(makeRfEdge({ sourceHandle: undefined, targetHandle: undefined }))
     expect(result.source_handle).toBeNull()