docs: rewrite threat-model.md for Soroban/Stellar-Save (#863)

[olaayoade27-coder]

Summary

Motivation

Closes #

Type of change

• feat — new feature
• fix — bug fix
• docs — documentation only
• refactor — code restructuring, no behaviour change
• test — tests only
• chore — build, deps, tooling
• perf — performance improvement
• Breaking change (existing behaviour changes)

Changes

Testing

Smart contract:

cargo test -p stellar-save

Frontend:

cd frontend && npm test run

Manual steps (if applicable):

Checklist

• cargo fmt run (Rust changes)
• cargo clippy -- -D warnings passes (Rust changes)
• npm run lint passes (frontend changes)
• Tests added or updated for new/changed behaviour
• CI is green
• Documentation updated (if behaviour changed)
• No secrets or .env files staged

Screenshots / recordings

pr closes #863

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