[contracts] Add oracle contract with price staleness detection as a security example

[CelestinaBeing]

Context

contracts/oracle/ exists. An oracle contract that demonstrates price staleness detection (reading stale prices as a vulnerability) would make a good S006 teaching example.

Work

• Add a get_price_unsafe(asset) function that reads without checking staleness (triggers S006)
• Add a get_price_safe(asset, max_age_ledgers) function that checks freshness
• Add set_price(asset, price, timestamp) admin function
• Write tests that verify staleness detection works correctly
• Add Sanctifier analysis showing S006 on the unsafe variant

Acceptance criteria

• sanctifier analyze contracts/oracle finds S006 on get_price_unsafe
• get_price_safe does not trigger S006
• Tests cover: fresh price, stale price, missing price