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/modals/__tests__/EdgeModal.test.tsx

@@ -165,6 +165,41 @@ describe('EdgeModal', () => {
     expect(onSubmit.mock.calls[0][0].animated).toBe('basic')
   })
 
+  // ── Arrow markers ─────────────────────────────────────────────────────────
+
+  it('arrows default to off', () => {
+    const onSubmit = vi.fn()
+    render(<EdgeModal open onClose={vi.fn()} onSubmit={onSubmit} />)
+    fireEvent.click(screen.getByRole('button', { name: 'Connect' }))
+    expect(onSubmit.mock.calls[0][0].marker_start).toBe(false)
+    expect(onSubmit.mock.calls[0][0].marker_end).toBe(false)
+  })
+
+  it('toggling End arrow sends marker_end: true', () => {
+    const onSubmit = vi.fn()
+    render(<EdgeModal open onClose={vi.fn()} onSubmit={onSubmit} />)
+    fireEvent.click(screen.getByRole('button', { name: /Arrow End/ }))
+    fireEvent.click(screen.getByRole('button', { name: 'Connect' }))
+    expect(onSubmit.mock.calls[0][0].marker_end).toBe(true)
+    expect(onSubmit.mock.calls[0][0].marker_start).toBe(false)
+  })
+
+  it('toggling Start arrow sends marker_start: true', () => {
+    const onSubmit = vi.fn()
+    render(<EdgeModal open onClose={vi.fn()} onSubmit={onSubmit} />)
+    fireEvent.click(screen.getByRole('button', { name: /Arrow Start/ }))
+    fireEvent.click(screen.getByRole('button', { name: 'Connect' }))
+    expect(onSubmit.mock.calls[0][0].marker_start).toBe(true)
+  })
+
+  it('pre-fills arrows from initial', () => {
+    const onSubmit = vi.fn()
+    render(<EdgeModal open onClose={vi.fn()} onSubmit={onSubmit} initial={{ marker_start: true, marker_end: true }} />)
+    fireEvent.click(screen.getByRole('button', { name: 'Connect' }))
+    expect(onSubmit.mock.calls[0][0].marker_start).toBe(true)
+    expect(onSubmit.mock.calls[0][0].marker_end).toBe(true)
+  })
+
   it('selecting None after Snake omits animated from payload', () => {
     const onSubmit = vi.fn()
     render(<EdgeModal open onClose={vi.fn()} onSubmit={onSubmit} />)