Add supply/borrow caps with per-user limits and global pool caps

[Smartdevs17]

Description

Implement configurable caps at both user and pool level to prevent concentration risk and ensure fair access to liquidity.

Acceptance Criteria

• Per-user supply cap (max percentage of pool)
• Per-user borrow cap (based on collateral)
• Global pool supply/borrow cap
• Cap enforcement with clear error messages
• Governance-controlled cap parameters
• Cap utilization monitoring dashboard

Technical Scope

• stellar-lend/contracts/lending/src/meta.rs
• stellar-lend/contracts/lending/src/meta_test.rs
• Edge: cap race condition on popular pools, whale cap bypass via multiple accounts