feat: selectable marker shapes per edge endpoint

Replace the on/off arrowhead toggle with a per-end shape picker. Each end
(start/end) independently selects: none, arrow, arrow-open, circle, diamond,
or square. Markers still recolor live from the resolved stroke color.

Frontend:
- MarkerShape type + edgeMarkers util (normalizeMarker, MARKER_GEOMETRY);
  legacy boolean coerces to 'arrow' on read.
- Per-shape <marker> inner geometry; symmetric shapes use fixed orient.
- MarkerShapePicker reused in EdgeModal and CustomStyleModal.
- Serializer normalizes to shape strings.

Backend:
- Edge marker columns Boolean -> String (default 'none'); TEXT migration.
- normalize_marker() + validators coerce legacy bool / unknown values.

ha-relevant: yes

backend/app/schemas/utils.py

@@ -7,3 +7,21 @@ def normalize_animated(v: object) -> str:
     if v in ('snake', 'flow', 'basic'):
         return str(v)
     return 'none'
+
+
+MARKER_SHAPES = {'none', 'arrow', 'arrow-open', 'circle', 'diamond', 'square'}
+
+
+def normalize_marker(v: object) -> str:
+    """Normalize an edge endpoint marker to a shape string.
+
+    Legacy saves stored a boolean (True = filled arrow); coerce those and any
+    unknown value to a valid MarkerShape ('none' when off/unknown).
+    """
+    if v is True or v == 1 or v == '1':
+        return 'arrow'
+    if v is False or v == 0 or v == '0' or v is None:
+        return 'none'
+    if isinstance(v, str) and v in MARKER_SHAPES:
+        return v
+    return 'none'