fix: prevent redundant upgrade governance collisions

[dreamgene]

Closes #150

Summary

• block re-initiating the same WASM hash while that hash is already the pending upgrade
• add a 7-day cooldown for hashes that were rejected when the timelock elapsed without enough votes to execute
• clear the cooldown entry if that same hash is eventually executed successfully

Testing

• cargo fmt --all
• cargo test -p predict-iq (currently blocked by unrelated pre-existing compile failures in other modules, including duplicate contract exports in lib.rs and mismatched types/imports in bets.rs, fees.rs, and voting.rs)

Notes

• this change preserves the current single global pending-upgrade model: different hashes are still blocked while any upgrade is pending
• rejection is recorded when execute_upgrade is attempted after timelock but fails majority requirements

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