Save the configuration to a TOML configuration file

[CGodiksen]

Closes #368 by updating the configuration manager so it now persists the configuration to a modelardb.toml configuration file in the root of the local data folder. This file is updated when changes are made to the configuration using the UpdateConfiguration action. If the file is changed manually, the changes are first applied when the server is restarted.

[Copilot]

Pull request overview

This PR implements configuration persistence for ModelarDB-RS by introducing a TOML-based configuration file (modelardb.toml) stored in the root of the local data folder. The configuration is now loaded from this file on server startup and automatically updated when changes are made through the UpdateConfiguration action, providing a persistent state across server restarts.

Key Changes:

• Configuration is now persisted to a modelardb.toml file that is created on first server startup
• Configuration updates via the UpdateConfiguration action are saved to disk
• Configuration structure refactored to separate serializable data from runtime state

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
docs/user/README.md	Documents the new modelardb.toml configuration file and its behavior
Cargo.toml	Adds toml and serde dependencies for configuration serialization
Cargo.lock	Updates lock file with new TOML-related dependencies (toml 0.9.8)
crates/modelardb_server/Cargo.toml	Adds toml and serde workspace dependencies to server crate
crates/modelardb_server/src/error.rs	Adds error variants for TOML serialization/deserialization failures
crates/modelardb_server/src/context.rs	Updates context initialization to use async try_new for configuration manager
crates/modelardb_server/src/storage/mod.rs	Changes direct field access to getter methods for thread configuration
crates/modelardb_server/src/remote.rs	Updates documentation and error handling for UpdateConfiguration action
crates/modelardb_server/src/configuration.rs	Core implementation: splits Configuration struct for serialization, implements file I/O, adds validation, updates all setters to persist changes, and includes comprehensive tests

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