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/db/models.py

@@ -86,8 +86,8 @@ class Edge(Base):
     custom_color: Mapped[str | None] = mapped_column(String)
     path_style: Mapped[str | None] = mapped_column(String)
     animated: Mapped[str] = mapped_column(String, nullable=False, default='none')
-    marker_start: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
-    marker_end: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    marker_start: Mapped[str] = mapped_column(String, nullable=False, default='none')
+    marker_end: Mapped[str] = mapped_column(String, nullable=False, default='none')
     source_handle: Mapped[str | None] = mapped_column(String)
     target_handle: Mapped[str | None] = mapped_column(String)
     waypoints: Mapped[list[dict[str, float]] | None] = mapped_column(JSON, nullable=True)