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/app/schemas/edges.py

@@ -15,6 +15,8 @@ class EdgeBase(BaseModel):
     custom_color: str | None = None
     path_style: str | None = None
     animated: str = 'none'
+    marker_start: bool = False
+    marker_end: bool = False
     source_handle: str | None = None
     target_handle: str | None = None
     waypoints: list[dict[str, float]] | None = None
@@ -37,6 +39,8 @@ class EdgeUpdate(BaseModel):
     custom_color: str | None = None
     path_style: str | None = None
     animated: str | None = None
+    marker_start: bool | None = None
+    marker_end: bool | None = None
     source_handle: str | None = None
     target_handle: str | None = None
     waypoints: list[dict[str, float]] | None = None