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/stores/__tests__/themeStore.test.ts

@@ -39,7 +39,7 @@ describe('themeStore', () => {
   it('setCustomStyle replaces the entire definition', () => {
     const def: CustomStyleDef = {
       nodes: { server: { borderColor: '#ff0000', borderOpacity: 1, bgColor: '#000000', bgOpacity: 1, iconColor: '#ff0000', iconOpacity: 1, width: 200, height: 80 } },
-      edges: { ethernet: { color: '#00ff00', opacity: 0.8, pathStyle: 'bezier', animated: 'none' } },
+      edges: { ethernet: { color: '#00ff00', opacity: 0.8, pathStyle: 'bezier', animated: 'none', arrowStart: false, arrowEnd: false } },
     }
     useThemeStore.getState().setCustomStyle(def)
     expect(useThemeStore.getState().customStyle.nodes.server?.borderColor).toBe('#ff0000')