Add metadata-precheck module to validate SFTP metadata excels upfront

[Aberdur]

PR Description

• Introduce a new CLI command metadata-precheck that scans remote SFTP folders (skipping _invalid_samples) and validates every metadata Excel before running download/wrapper
• Add SchemaMapper helper to normalise headers, map rows to schema properties, and reconcile enum values without relying on ontologies in the spreadsheet
• For each row, run full JSON Schema validation (same rules as validate) so structural/enum/type errors are detected early
• Produce per-lab/per-file reports showing valid/invalid files, invalid sample counts, detailed messages, JSON summary and optional Excel export
• Print a Rich table with the top issues per lab to highlight common problems immediately
• Document the new command in the README (usage, options, rationale) so users know to run it before processing batches

Example

PR Checklist

• This comment contains a description of changes (with reason).
• Make sure your code lints (black and flake8).
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).

[saramonzon]

Docstrings are left, use same docstring format than other modules

SchemaMapper can be used in other modules? Should it be put in utils?

[saramonzon]

in the code should be no labels, only properties