Add flash loan manipulation guards

[TUPM96]

Closes #379

Summary

• add configurable flash-loan security limits for loan/liquidity, price impact, and TWAP deviation
• track per-asset active flash loans and latest security snapshots
• block liquidation pricing while debt/collateral asset is in an active flash loan
• add tests covering liquidity caps, excessive price impact, and TWAP liquidity manipulation simulations

Tests

• cargo fmt -p stellarlend-lending
• cargo check -p stellarlend-lending --lib
• cargo test -p stellarlend-lending -- --test-threads=4

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