Support response headers via JSON environment variable

[aram356]

Summary

• Add custom deserializer map_from_obj_or_str to parse response_headers from a JSON string env var, preserving header name casing and hyphens
• Add test verifying JSON env var override works correctly
• Update TOML config comments and docs with the JSON env var syntax

Closes #319

Details

The config crate lowercases env var keys and converts separators, so TRUSTED_SERVER__RESPONSE_HEADERS__X_ROBOTS_TAG becomes x_robots_tag. This makes it impossible to set headers like X-Robots-Tag via individual env vars.

The fix adds a custom serde deserializer (following the existing vec_from_seq_or_map pattern) that accepts a JSON object string:

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

The existing TOML [response_headers] table format is unaffected.

Test plan

• New test test_response_headers_override_with_json_env passes
• All 352 existing tests pass (cargo test --workspace)
• Manual verification: set the env var and confirm headers appear in responses

[ChristianPavilonis]

Looks good once verified.