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.

docs/akkudoktoreos/serverapi.rst

@@ -0,0 +1,17 @@
+..
+    SPDX-License-Identifier: Apache-2.0
+    File has to be of RST format to make openapi directive work correctly
+
+.. _akkudoktoreos_server_api:
+
+Server API
+##########
+
+For a more detailed documentation see the Swagger interface: `EOS OpenAPI Spec <https://petstore3.swagger.io/?url=https://raw.githubusercontent.com/Akkudoktor-EOS/EOS/refs/heads/main/openapi.json>`_
+
+.. openapi:: openapi.json
+   :examples:
+
+..
+    Due to bugs in sphinxcontrib-openapi referenced request/response objects fail to render and anyOf is broken too.
+    :request: