Update backend to v0.3.6

[tripledoublev]

Summary

• update save-dweb-backend to v0.3.6 and Veilid to v0.5.3
• keep the save-rust public interface unchanged; Android JNI symbols, server entry points, route paths, and request/response structs are not changed
• avoid blocking group refresh on local writable repos by listing those files locally instead of attempting peer download

Public interface check

• no diff in android_bridge.rs, jni_globals.rs, server.rs, repos.rs, media.rs, models.rs, or utils.rs versus main
• whitespace-ignored source diff is limited to the internal refresh_group logic in src/groups.rs

Validation

• cargo check
• cargo test --no-run
• cargo clippy --all-targets --all-features -- -D warnings