Implement collateral asset whitelisting and RBAC hardening

[Dopezapha]

Closes #16

"### Objective
Restrict betting transactions to only use authorized collateral assets (e.g., XLM, USDC), preventing market volume manipulation using worthless 'spam' tokens.

Key Changes:

• Collateral Whitelisting: Implemented `WhitelistedToken` storage and enforced strict validation in [place_bet].
• RBAC Hardening: Unified all administrative operations (whitelisting, fees, global status) under the `access` module's `Role::Admin` checks.
• Governance Logic Synchronization: Properly implemented the 24-hour liveness window for market resolutions in both code and tests.
• Test Suite Stabilization:
  • Fixed 10+ integration test failures caused by the move to real Stellar Asset Contracts (SAC).
  • Overhauled [setup()] and [setup_market_with_winners()] to automatically handle token provisioning and whitelisting.
  • Achieved a 100% pass rate (57/57 tests) for all platform logic.

Impact:

• Protects the platform from spam-token manipulation.
• Hardens the administrative security model for production deployment.
• Ensures a stable, production-ready foundation for decentralized market governance."

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