REST: Allow setting single config value

* /v1/config/{path} supports setting single config value (post body). Lists are
   supported as well by using the index:
    - general/latitude (value: 55.55)
    - optimize/ev_available_charge_rates_percent/0 (value: 42)

   Whole tree can be overriden as well (no merge):
    - optimize/ev_available_charge_rates_percent (value: [42, 43, 44]

 * ConfigEOS: Add set_config_value, get_config_value

src/akkudoktoreos/config/config.py

@@ -30,7 +30,7 @@ from akkudoktoreos.core.coreabc import SingletonMixin
 from akkudoktoreos.core.decorators import classproperty
 from akkudoktoreos.core.logging import get_logger
 from akkudoktoreos.core.logsettings import LoggingCommonSettings
-from akkudoktoreos.core.pydantic import merge_models
+from akkudoktoreos.core.pydantic import access_nested_value, merge_models
 from akkudoktoreos.devices.settings import DevicesCommonSettings
 from akkudoktoreos.measurement.measurement import MeasurementCommonSettings
 from akkudoktoreos.optimization.optimization import OptimizationCommonSettings
@@ -379,6 +379,32 @@ class ConfigEOS(SingletonMixin, SettingsEOSDefaults):
         """
         self._setup()
 
+    def set_config_value(self, path: str, value: Any) -> None:
+        """Set a configuration value based on the provided path.
+
+        Supports string paths (with '/' separators) or sequence paths (list/tuple).
+        Trims leading and trailing '/' from string paths.
+
+        Args:
+            path (str): The path to the configuration key (e.g., "key1/key2/key3" or key1/key2/0).
+            value (Any]): The value to set.
+        """
+        access_nested_value(self, path, True, value)
+
+    def get_config_value(self, path: str) -> Any:
+        """Get a configuration value based on the provided path.
+
+        Supports string paths (with '/' separators) or sequence paths (list/tuple).
+        Trims leading and trailing '/' from string paths.
+
+        Args:
+            path (str): The path to the configuration key (e.g., "key1/key2/key3" or key1/key2/0).
+
+        Returns:
+            Any: The retrieved value.
+        """
+        return access_nested_value(self, path, False)
+
     def _create_initial_config_file(self) -> None:
         if self.general.config_file_path and not self.general.config_file_path.exists():
             self.general.config_file_path.parent.mkdir(parents=True, exist_ok=True)

src/akkudoktoreos/core/pydantic.py

@@ -51,6 +51,70 @@ def merge_models(source: BaseModel, update_dict: dict[str, Any]) -> dict[str, An
     return merged_dict
 
 
+def access_nested_value(
+    model: BaseModel, path: str, setter: bool, value: Optional[Any] = None
+) -> Any:
+    """Get or set a nested model value based on the provided path.
+
+    Supports string paths (with '/' separators) or sequence paths (list/tuple).
+    Trims leading and trailing '/' from string paths.
+
+    Args:
+        model (BaseModel): The model object for partial assignment.
+        path (str): The path to the model key (e.g., "key1/key2/key3" or key1/key2/0).
+        setter (bool): True to set value at path, False to return value at path.
+        value (Optional[Any]): The value to set.
+
+    Returns:
+        Any: The retrieved value if acting as a getter, or None if setting a value.
+    """
+    path_elements = path.strip("/").split("/")
+
+    cfg: Any = model
+    parent: BaseModel = model
+    model_key: str = ""
+
+    for i, key in enumerate(path_elements):
+        is_final_key = i == len(path_elements) - 1
+
+        if isinstance(cfg, list):
+            try:
+                idx = int(key)
+                if is_final_key:
+                    if not setter:  # Getter
+                        return cfg[idx]
+                    else:  # Setter
+                        new_list = list(cfg)
+                        new_list[idx] = value
+                        # Trigger validation
+                        setattr(parent, model_key, new_list)
+                else:
+                    cfg = cfg[idx]
+            except ValidationError as e:
+                raise ValueError(f"Error updating model: {e}") from e
+            except (ValueError, IndexError) as e:
+                raise IndexError(f"Invalid list index at {path}: {key}") from e
+
+        elif isinstance(cfg, BaseModel):
+            parent = cfg
+            model_key = key
+            if is_final_key:
+                if not setter:  # Getter
+                    return getattr(cfg, key)
+                else:  # Setter
+                    try:
+                        # Verification also if nested value is provided opposed to just setattr
+                        # Will merge partial assignment
+                        cfg = cfg.__pydantic_validator__.validate_assignment(cfg, key, value)
+                    except Exception as e:
+                        raise ValueError(f"Error updating model: {e}") from e
+            else:
+                cfg = getattr(cfg, key)
+
+        else:
+            raise KeyError(f"Key '{key}' not found in model.")
+
+
 class PydanticTypeAdapterDateTime(TypeAdapter[pendulum.DateTime]):
     """Custom type adapter for Pendulum DateTime fields."""
 

src/akkudoktoreos/server/eos.py

@@ -10,7 +10,9 @@ from typing import Annotated, Any, AsyncGenerator, Dict, List, Optional, Union
 
 import httpx
 import uvicorn
-from fastapi import FastAPI, Query, Request
+from fastapi import Body, FastAPI
+from fastapi import Path as FastapiPath
+from fastapi import Query, Request
 from fastapi.exceptions import HTTPException
 from fastapi.responses import FileResponse, HTMLResponse, RedirectResponse, Response
 
@@ -302,6 +304,58 @@ def fastapi_config_put(settings: SettingsEOS) -> ConfigEOS:
     return config_eos
 
 
+@app.put("/v1/config/{path:path}", tags=["config"])
+def fastapi_config_put_key(
+    path: str = FastapiPath(
+        ..., description="The nested path to the configuration key (e.g., general/latitude)."
+    ),
+    value: Any = Body(..., description="The value to assign to the specified configuration path."),
+) -> ConfigEOS:
+    """Update a nested key or index in the config model.
+
+    Args:
+        path (str): The nested path to the key (e.g., "general/latitude" or "optimize/nested_list/0").
+        value (Any): The new value to assign to the key or index at path.
+
+    Returns:
+        configuration (ConfigEOS): The current configuration after the update.
+    """
+    try:
+        config_eos.set_config_value(path, value)
+    except IndexError as e:
+        raise HTTPException(status_code=400, detail=str(e))
+    except KeyError as e:
+        raise HTTPException(status_code=404, detail=str(e))
+    except Exception as e:
+        raise HTTPException(status_code=400, detail=str(e))
+
+    return config_eos
+
+
+@app.get("/v1/config/{path:path}", tags=["config"])
+def fastapi_config_get_key(
+    path: str = FastapiPath(
+        ..., description="The nested path to the configuration key (e.g., general/latitude)."
+    ),
+) -> Response:
+    """Get the value of a nested key or index in the config model.
+
+    Args:
+        path (str): The nested path to the key (e.g., "general/latitude" or "optimize/nested_list/0").
+
+    Returns:
+        value (Any): The value of the selected nested key.
+    """
+    try:
+        return config_eos.get_config_value(path)
+    except IndexError as e:
+        raise HTTPException(status_code=400, detail=str(e))
+    except KeyError as e:
+        raise HTTPException(status_code=404, detail=str(e))
+    except Exception as e:
+        raise HTTPException(status_code=400, detail=str(e))
+
+
 @app.get("/v1/measurement/keys", tags=["measurement"])
 def fastapi_measurement_keys_get() -> list[str]:
     """Get a list of available measurement keys."""