Add recursive child inscription lookup by index

[0xjc65eth]

Summary

• add /r/children/{inscription_id}/inscriptions/at/{index}
• support positive indexes from the first child and negative indexes from the latest child
• return Cache-Control: no-store for negative lookups because the latest child can change
• add regression coverage for empty, first, and latest-child lookups

Why
Consumers currently need to walk all child inscription pages to find the latest child. This adds the indexed lookup suggested in #3923 while matching the existing /r/sat/{sat}/at/{index} convention.

Testing

• cargo +1.93.0 test child_inscriptions_recursive_endpoint
• cargo +1.93.0 fmt -- --check
• cargo +1.93.0 check

Closes #3923.