Refactor liquidation strategy into pluggable module system

[Smartdevs17]

Context

Liquidation logic is monolithic and needs modularization.

Current Limitation

All liquidation logic is in a single module.

Expected Outcome

• Pluggable liquidation strategies
• Strategy selection based on conditions
• Strategy testing framework
• Strategy analytics

Acceptance Criteria

• Liquidation strategy interface
• Multiple strategy implementations
• Strategy selection algorithm
• Strategy testing framework
• Strategy performance comparison
• Strategy configuration API
• Strategy documentation

Technical Scope

• stellar-lend/contracts/hello-world/src/liquidate.rs - Liquidation module
• stellar-lend/contracts/lending/src/ - Modular liquidation
• api/src/routes/liquidation.ts - Liquidation API
• tests/ - Strategy tests