[Kapozux]

Fixes #1456.

Datasource uploads accept CSV extensions case-insensitively, but the readers and the legacy upload endpoint selected the Excel parser for stored names such as orders.CSV. Use the same case-insensitive CSV check for preview, import, CSV sheet naming and the legacy upload endpoint. Keep the original filename and path so existing stored uploads work too.

Verification

• Added eight regressions covering identical .csv, .CSV and .CsV files, an Excel workbook named sales.CSV.xlsx, and a multipart request to the legacy upload route. Five failed on the base commit. All eight pass with this change.
• Exercised /datasource/parseExcel, /datasource/importToDb and /datasource/uploadExcel with FastAPI TestClient, test identity and a disposable PostgreSQL 17.11 database. Each suffix produced the same two preview rows, the import returned Sheet1, and both import paths persisted the expected rows.
• Ran the CSV, upload path, import path and upload filename test files with the same environment and tests on both revisions. Base 2e81ded5 had 66 passes and 14 failures. This branch has 71 passes and 9 failures. The remaining nine are unchanged upload guard failures: seven expect a top-level filePath and two expect a different rejection detail for orders.xlsx:extra.