refactor: unify internal timestamp casting to u64

[dreamgene]

Closes #157

Summary

• unify oracle timestamp handling to u64 internally
• add safe conversion for external i64 publish times using try_into()
• centralize age validation behind an is_stale helper using saturating_sub
• reject invalid negative external timestamps explicitly

Testing

• cargo fmt --all
• cargo test -p predict-iq modules::oracles_test -- --nocapture (blocked by unrelated pre-existing compile failures in other modules such as lib.rs, bets.rs, fees.rs, and voting.rs before oracle tests can run)

Notes

• this change also restores the oracle config shape expected by the repo and wires the oracle test module into the crate test tree
• no raw as u64 casts remain for external oracle publish times in the oracle module

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