Add FastAPI interface between the game and map.

A possible explanation for the infrequent CTDs we've been seeing since
adding fast forward is that QWebChannel doesn't keep a reference to the
python objects that it passes to js, so if the object is GC'd before the
front end is done with it, it crashes.

We don't really like QWebChannel anyway, so this begins replacing that
with FastAPI.

game/server/leaflet.py

@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+from typing import Union
+
+from dcs import Point
+from shapely.geometry import LineString, MultiLineString, MultiPolygon, Polygon
+
+from game.theater import ConflictTheater
+
+LeafletLatLon = list[float]
+LeafletPoly = list[LeafletLatLon]
+
+
+class ShapelyUtil:
+    @staticmethod
+    def poly_to_leaflet(poly: Polygon, theater: ConflictTheater) -> LeafletPoly:
+        if poly.is_empty:
+            return []
+        return [
+            theater.point_to_ll(Point(x, y)).as_list() for x, y in poly.exterior.coords
+        ]
+
+    @classmethod
+    def polys_to_leaflet(
+        cls, poly: Union[Polygon, MultiPolygon], theater: ConflictTheater
+    ) -> list[LeafletPoly]:
+        if isinstance(poly, MultiPolygon):
+            polys = poly.geoms
+        else:
+            polys = [poly]
+        return [cls.poly_to_leaflet(poly, theater) for poly in polys]
+
+    @staticmethod
+    def line_to_leaflet(
+        line: LineString, theater: ConflictTheater
+    ) -> list[LeafletLatLon]:
+        return [theater.point_to_ll(Point(x, y)).as_list() for x, y in line.coords]
+
+    @classmethod
+    def lines_to_leaflet(
+        cls, line_string: MultiLineString | LineString, theater: ConflictTheater
+    ) -> list[list[LeafletLatLon]]:
+        if isinstance(line_string, MultiLineString):
+            lines = line_string.geoms
+        else:
+            lines = [line_string]
+        return [cls.line_to_leaflet(line, theater) for line in lines]