Support response headers via environment variable with proper casing

[aram356]

Problem

The [response_headers] config supports setting custom HTTP response headers via TOML, but setting them via environment variables loses header name casing and hyphens.

For example, TRUSTED_SERVER__RESPONSE_HEADERS__X_ROBOTS_TAG="noindex" results in the header key x_robots_tag instead of X-Robots-Tag, because the config crate lowercases env var names and can't represent hyphens.

Solution

Support a JSON object string as the env var value:

TRUSTED_SERVER__RESPONSE_HEADERS='{"X-Robots-Tag": "noindex", "X-Custom-Header": "custom value"}'

This preserves exact header name casing and hyphens. The existing TOML [response_headers] table format continues to work unchanged.

Acceptance Criteria

• Custom deserializer handles both TOML table and JSON string inputs
• Header names preserve casing and hyphens when set via env var
• Existing TOML-based config is unaffected
• Test coverage for JSON env var override
• Documentation updated