feat: add redirect grace period

[dangelo352]

Summary

• add grace_period: u64 to ProductivityVault and create_vault
• bound grace periods to MAX_VAULT_DURATION while preserving grace_period == 0 legacy redirect behavior
• require redirect_funds to wait until now > end_timestamp + grace_period using checked addition
• cover exact-boundary rejection, after-boundary success, oversized grace rejection, overflow rejection, and proptest storage/bounds
• update README, backend docs, contract interface, and add docs/GRACE_PERIOD.md

Tests

• cargo test --test lifecycle --test proptest_timestamps -- --nocapture
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --check
• git diff --check

Closes #222

[1nonlypiece]

hey, good work, but this can't go in until you're assigned to #222 — that's our process to avoid two people on the same thing. closing for now; claim the issue first and re-raise. https://discord.gg/xvNAvMJf