Deep-Merge Configuration Dicts

Apps layer configuration: defaults, then an environment file, then command-line overrides. A shallow {**a, **b} replaces whole nested sections. You need a deep merge.

Implement deep_merge(base, override), returning a new dict:

• Keys only in one input are copied over.
• If both values are dicts, merge them recursively.
• Otherwise the override's value wins, including lists, which are replaced rather than concatenated.
• An override value of None deletes the key.
• Neither input may be modified, and nested dicts in the result must not be the same objects as in the inputs.