Add burn height to inscription API

[0xjc65eth]

Expose the block height where a burned inscription was mined by adding burn_height to inscription JSON responses. The field is null for non-burned inscriptions and set from the burn transaction block header when the inscription has the burned charm.

Closes #4431.

Tests:

• cargo +1.93.0 test get_burned_inscription_includes_burn_height --test integration
• cargo +1.93.0 test json_api --test integration
• cargo +1.93.0 fmt -- --check
• cargo +1.93.0 check
• git diff --check