[pitill0]

Summary

Fix incorrect Content-Length: 0 responses for HEAD / in FluxTuner Web.

The index endpoint now reports the size of the corresponding GET representation while continuing to return an empty body for HEAD requests.

Changes

• Explicitly set Content-Length for successful HEAD responses.
• Preserve identity and gzip response representations.
• Maintain existing ETags, Vary: Accept-Encoding, and cache behavior.
• Preserve conditional HEAD responses (304 Not Modified) without a body or Content-Length.
• Add parametrized regression tests covering both legacy and versioned static asset modes.

Validation

Final pre-PR gate: PASS

• Ruff lint and format: passed.
• Mypy: passed (117 source files).
• Bandit: no issues identified.
• pip-audit: no known vulnerabilities in audited dependencies.
• JavaScript syntax: 40 files passed.
• pytest: 1,873 passed, 8 skipped.
• Wheel and sdist: built and verified.
• Git working tree: clean.

Scope

This PR fixes the HTTP HEAD contract only. It does not change static asset publication, caching policies, favicon resources, or JavaScript loading behavior.

No production deployment or configuration changes are included.