…tting

The module documents environment variables as taking precedence over `datajoint.json`, but `_update_from_flat_dict` enforced that only for keys in the hand-kept `ENV_VAR_MAPPING`. For every other setting the file replaced the value read from the environment: `jobs.*` (`DJ_JOBS_*`), `provenance.*` (`DJ_PROVENANCE_*`), `database.use_tls`, `database.reconnect`, and the top-level `safemode`, `query_cache`, `download_path` and `ignore_config_file`. A deployment could not, for example, turn off `safemode` or provenance capture for one worker when the project's file set it.

`ENV_VAR_MAPPING` is now derived from the settings models: a field's variable is its `validation_alias`, otherwise its model's `env_prefix` plus the field name. A setting added later is covered as soon as it is declared, including one that has only a `validation_alias`. Models without an `env_prefix` (`connection`, `display`) contribute only their aliased fields, matching the documented variables, and `stores` stays with `_apply_stores_env`, which already runs after the file. The eleven names in the old mapping are unchanged.

The tests list every setting that reads an environment variable, assert the derived mapping matches that list, and check for each one, in both flat and nested file layouts, that pydantic reads the variable and the file does not replace it.

Closes #1585.