Fix review findings and quality gates

[zerox80]

Summary

• fix guest comment rate limiting by storing and querying a stable rate_limit_key
• make admin tutorial comments use authenticated claims instead of trusting author payloads
• align admin upload body limits with 10MB uploads plus multipart overhead
• clean up clippy/eslint findings, duplicate form data, dynamic icon rendering, and test harness setup

Verification

• cargo fmt --manifest-path backend/Cargo.toml
• cargo clippy --manifest-path backend/Cargo.toml --all-targets -- -D warnings
• cargo test --manifest-path backend/Cargo.toml -j 1
• npm.cmd run lint
• npm.cmd test -- --run --pool forks --maxWorkers=1
• npm.cmd run build
• git diff --check