feat: import quotations from CSV and Excel

[yigitcan-ozturk]

What changed

• add CSV and XLSX quotation ingestion alongside existing JSON inputs
• allow one tabular file to contain multiple supplier quotations
• normalize and validate tabular numeric fields before scoring
• expose load_quotes and validate_quote through the public Python API
• add openpyxl as the Excel runtime dependency
• add CSV/XLSX tests across the existing Python 3.11–3.13 CI matrix
• add a tabular sample and document the new workflows

Compatibility

• existing one-JSON-file-per-supplier CLI usage remains supported
• scoring weights and the v0.2 JSON output contract are unchanged
• mixed currencies are still rejected and remain the responsibility of currency-normalizer

Why

This closes the first roadmap gap between the deterministic scoring engine and real procurement workflows, where quotations are commonly maintained in spreadsheets.