Add source mapping configuration to sources page

[jal347]

This pull request enhances how source field mappings are handled and exposed in metadata, especially for multi-uploader sources. It ensures that per-source field mappings are merged, properly formatted, and only exposed in development mode, while also adding field-to-source annotations in the /metadata/fields endpoint.

Source mapping handling and exposure:

• Merges all sub-source field mappings into a single mapping at the main source level (_meta.src.<source>.mapping) during metadata collection, ensuring multi-uploader sources have a unified mapping. [1] [2]
• Exposes per-source field mappings only in development mode (/metadata?dev), flattening them to match the /metadata/fields representation, and strips them from public responses.
• Adds a utility method _transform_source_mappings to safely apply transformations (e.g., flattening or removing mappings) to each source's mapping without mutating cached metadata.

Field annotation improvements:

• Annotates each field in the /metadata/fields response with the main source(s) that define it, based on the merged and flattened per-source mappings, making data provenance clearer for consumers.

Imports and dependencies:

• Adds necessary imports for merge_struct and copy to support the new merging and deep-copy logic. [1] [2]