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/canvas.py

@@ -4,7 +4,7 @@ from pydantic import BaseModel, field_validator
 
 from app.schemas.edges import EdgeResponse
 from app.schemas.nodes import NodeResponse
-from app.schemas.utils import normalize_animated
+from app.schemas.utils import normalize_animated, normalize_marker
 
 
 class NodeSave(BaseModel):
@@ -52,8 +52,8 @@ class EdgeSave(BaseModel):
     custom_color: str | None = None
     path_style: str | None = None
     animated: str = 'none'
-    marker_start: bool = False
-    marker_end: bool = False
+    marker_start: str = 'none'
+    marker_end: str = 'none'
     source_handle: str | None = None
     target_handle: str | None = None
     waypoints: list[dict[str, float]] | None = None
@@ -63,6 +63,11 @@ class EdgeSave(BaseModel):
     def validate_animated(cls, v: object) -> str:
         return normalize_animated(v)
 
+    @field_validator('marker_start', 'marker_end', mode='before')
+    @classmethod
+    def validate_marker(cls, v: object) -> str:
+        return normalize_marker(v)
+
 
 class CanvasSaveRequest(BaseModel):
     nodes: list[NodeSave] = []