chore: migrate rust/canister-snapshots to icp-cli

[marc0olo]

Summary

• Replace dfx.json with icp.yaml using @dfinity/rust@v3.3.0
• Move Rust source from src/chat/ to backend/, rename package to backend
• Update procedure macros from ic-cdk-macros to ic-cdk 0.20 (#[ic_cdk::update], etc.)
• Add Makefile with test target exercising the full snapshot create/restore lifecycle
• Rewrite README.md with icp-cli instructions, preserving the key insight that snapshots must be taken/restored while the canister is stopped
• Add CI workflow (.github/workflows/canister-snapshots.yml) using icp-dev-env-rust:1.0.0
• Delete legacy dfx workflow (rust-canister-snapshots-example.yaml)

Test plan

• icp network start -d && icp deploy succeeds
• make test passes: appends a message, creates snapshot, triggers data loss via remove_spam bug, restores snapshot, verifies message is recovered
• CI workflow runs on PR and passes

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