Migrate from Flask to FastAPI (#163)

* Migrate from Flask to FastAPI

 * FastAPI migration:
    - Use pydantic model classes as input parameters to the
      data/calculation classes.
    - Interface field names changed to constructor parameter names (for
      simplicity only during transition, should be updated in a followup
      PR).
    - Add basic interface requirements (e.g. some values > 0, etc.).
 * Update tests for new data format.
 * Python requirement down to 3.9 (TypeGuard no longer needed)
 * Makefile: Add helpful targets (e.g. development server with reload)

* Move API doc from README to pydantic model classes (swagger)

 * Link to swagger.io with own openapi.yml.
 * Commit openapi.json and check with pytest for changes so the
   documentation is always up-to-date.

* Streamline docker

* FastAPI: Run startup action on dev server

 * Fix config for /strompreis, endpoint still broken however.

* test_openapi: Compare against docs/.../openapi.json

* Move fastapi to server/ submodule

 * See #187 for new repository structure.

tests/generate_openapi.py

@@ -0,0 +1,25 @@
+import json
+from pathlib import Path
+
+from fastapi.openapi.utils import get_openapi
+
+from akkudoktoreos.server.fastapi_server import app
+
+
+def generate_openapi(filename: str | Path = "openapi.json"):
+    with open(filename, "w") as f:
+        json.dump(
+            get_openapi(
+                title=app.title,
+                version=app.version,
+                openapi_version=app.openapi_version,
+                description=app.description,
+                routes=app.routes,
+            ),
+            f,
+            indent=2,
+        )
+
+
+if __name__ == "__main__":
+    generate_openapi()