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/db/database.py

@@ -81,6 +81,10 @@ async def init_db() -> None:
             await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN target_handle TEXT")
         with suppress(OperationalError):
             await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN animated BOOLEAN NOT NULL DEFAULT 0")
+        with suppress(OperationalError):
+            await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN marker_start BOOLEAN NOT NULL DEFAULT 0")
+        with suppress(OperationalError):
+            await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN marker_end BOOLEAN NOT NULL DEFAULT 0")
         with suppress(OperationalError):
             await conn.exec_driver_sql("ALTER TABLE nodes ADD COLUMN cpu_count INTEGER")
         with suppress(OperationalError):