[SC-27] Implement valuation oracle module for tokenized assets

[yusuftomilola]

Overview

Implement a valuation oracle module that stores and updates the on-chain market value of tokenized assets inside contracts/contrib/.

Location

All work must be inside contracts/contrib/src/oracle.rs.

Acceptance Criteria

• ValuationEntry struct: asset_id, value, currency, source, timestamp
• update_valuation(env, asset_id, value, currency) authorized oracle only; stores entry and appends to history
• get_latest_valuation(env, asset_id) -> ValuationEntry panics if none exists
• get_valuation_history(env, asset_id) -> Vec<ValuationEntry> returns last 10 entries
• add_oracle(env, oracle) and remove_oracle(env, oracle) admin only
• Emits event on each update