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

backend/tests/test_edges.py

@@ -91,6 +91,31 @@ async def test_update_edge_custom_color_and_path_style(client: AsyncClient, head
     assert res.json()["path_style"] == "smooth"
 
 
+async def test_create_edge_with_arrow_markers(client: AsyncClient, headers: dict, two_nodes):
+    src, tgt = two_nodes
+    res = await client.post("/api/v1/edges", json={"source": src, "target": tgt, "type": "ethernet", "marker_start": True, "marker_end": True}, headers=headers)
+    assert res.status_code == 201
+    assert res.json()["marker_start"] is True
+    assert res.json()["marker_end"] is True
+
+
+async def test_create_edge_defaults_arrow_markers_off(client: AsyncClient, headers: dict, two_nodes):
+    src, tgt = two_nodes
+    res = await client.post("/api/v1/edges", json={"source": src, "target": tgt, "type": "ethernet"}, headers=headers)
+    assert res.status_code == 201
+    assert res.json()["marker_start"] is False
+    assert res.json()["marker_end"] is False
+
+
+async def test_update_edge_arrow_markers(client: AsyncClient, headers: dict, two_nodes):
+    src, tgt = two_nodes
+    edge_id = (await client.post("/api/v1/edges", json={"source": src, "target": tgt, "type": "ethernet"}, headers=headers)).json()["id"]
+    res = await client.patch(f"/api/v1/edges/{edge_id}", json={"marker_end": True}, headers=headers)
+    assert res.status_code == 200
+    assert res.json()["marker_end"] is True
+    assert res.json()["marker_start"] is False
+
+
 async def test_create_edge_requires_auth(client: AsyncClient, two_nodes):
     src, tgt = two_nodes
     res = await client.post("/api/v1/edges", json={"source": src, "target": tgt, "type": "ethernet"})