fix: remove chunked database download header

[suzunn]

Problem

Database downloads include both a Content-Length header and an explicit Transfer-Encoding: chunked header. Browsers do not show download progress for that response shape, even though the file size is already known.

Closes #2663.

Solution

I removed the manual Transfer-Encoding: chunked header from immutable database downloads and updated the existing download test to assert that the response keeps Content-Length without setting transfer-encoding.

Testing

• uv run ruff check datasette\views\database.py tests\test_html.py
• uv run black --check datasette\views\database.py tests\test_html.py
• git diff --check

I also ran uv run pytest tests\test_html.py -k database_download_for_immutable -q. The response assertions completed, but the test fails during Windows temporary directory cleanup with PermissionError: [WinError 32] The process cannot access the file because it is being used by another process; that looks like the existing Windows file-handle cleanup issue tracked separately from this header change.