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/types/index.ts

@@ -169,6 +169,10 @@ export interface EdgeData extends Record<string, unknown> {
   custom_color?: string
   path_style?: EdgePathStyle
   animated?: boolean | 'snake' | 'flow' | 'basic' | 'none'
+  /** Filled arrowhead at the source end. */
+  marker_start?: boolean
+  /** Filled arrowhead at the target end. */
+  marker_end?: boolean
   waypoints?: Waypoint[]
 }
 
@@ -257,6 +261,10 @@ export interface EdgeTypeStyle {
   opacity: number
   pathStyle: EdgePathStyle
   animated: 'none' | 'snake' | 'flow' | 'basic'
+  /** Default filled arrowhead at the source end for new edges of this type. */
+  arrowStart: boolean
+  /** Default filled arrowhead at the target end for new edges of this type. */
+  arrowEnd: boolean
 }
 
 export interface CustomStyleDef {