Migrate pool configuration to on-chain governance parameter store

[Smartdevs17]

Context

Pool configuration needs to be managed through governance.

Current Limitation

Pool configuration is off-chain or hardcoded.

Expected Outcome

• On-chain parameter store
• Governance-controlled parameter updates
• Parameter versioning
• Parameter impact simulation

Acceptance Criteria

• On-chain parameter store contract
• Governance voting for parameter changes
• Parameter versioning with history
• Parameter impact simulation
• Parameter validation rules
• Parameter change notifications
• Parameter documentation

Technical Scope

• stellar-lend/contracts/hello-world/src/governance.rs - Governance
• New contract: stellar-lend/contracts/parameters/ - Parameter store
• api/src/routes/parameters.ts - Parameter API
• frontend/ - Parameter management UI