remove luminance collection and calculation; it was minimally useful

[bghira]

This pull request removes all code related to image luminance calculation, storage, and logging throughout the codebase. The luminance feature is no longer used in data processing, metadata, training, validation, or the user interface. This simplifies the code and reduces unnecessary computation and storage.

The most important changes are:

Removal of luminance calculation and usage:

• Deleted the entire brightness.py module, which contained all functions for calculating image luminance, including batch and multiprocessing support.
• Removed all calls to and imports of calculate_luminance and related batch luminance methods from image processing, metadata backends, training, and validation code. [1] [2] [3] [4]
• Eliminated all code that calculated, stored, or updated luminance in batch processing and metadata, including database updates and per-batch calculations. [1] [2] [3] [4] [5]

Removal from data structures and user interface:

• Removed the luminance field from all batch, metadata, and data model classes, as well as from the dataset viewer UI and API. [1] [2] [3] [4] [5]
• Removed luminance from logging and reporting in training and validation, including Weights & Biases (wandb) logs and tables. [1] [2] [3] [4] [5] [6]

Test cleanup:

• Updated or removed test fixtures, mocks, and assertions that referenced luminance values or batch luminance calculation. [1] [2] [3] [4] [5] [6]