fix(share-pnl): pin leverage to lease-open snapshot

[metodi96]

Summary

Fix the share-pnl card's leverage calculation, which produced wildly inflated values for short positions (e.g. x12.3 displayed on a real 1.7x position) due to a dimensional mismatch between USD and LPN units. Pin the displayed value to the lease-open snapshot so it matches what the user actually opened at and doesn't drift with interest accrual or manual repayments.

Changes

Backend

• backend/src/external/etl.rs — adds loan_amount_stable field on EtlLeaseInfo (renames from ETL's LS_loan_amnt_stable, the loan principal in stable USD at lease open).
• backend/src/handlers/leases.rs — adds loan_amount_stable to LeaseEtlData and propagates it through the etl_info mapping.

Frontend

• src/common/api/types/leases.ts — adds loan_amount_stable?: string to LeaseEtlData.
• src/common/utils/LeaseCalculator.ts — adds leverageAtOpen: Dec | null to LeaseDisplayData; calculateDisplayData computes (downPayment + loanAtOpen) / downPayment from the frozen lease-open snapshot. Both fields are in stable USDC decimals (6), so the units line up across longs and shorts.
• src/modules/leases/components/single-lease/SharePnLDialog.vue — leverageMultiple prefers leaseDisplayData.leverageAtOpen. Falls back to a live computation using totalDebtUsd (instead of the previous totalDebt) when ETL hasn't exposed loan_amount_stable yet; this also fixes the dimensional bug in the fallback path for older leases.

Root cause

The previous leverageMultiple formula was (downPayment + totalDebt) / downPayment. downPayment came from parseEtlData and is denominated in USD (LS_cltr_amnt_stable: asset_micro_units × price, divided by collateral decimals). totalDebt came from calculateDebt and is denominated in LPN units (new Dec(lease.debt.principal, lpnDecimals)).

For longs the LPN is USDC ≈ $1, so the LPN-quantity ≈ USD value and the formula accidentally worked. For shorts the LPN is the volatile asset — on the OSMO short we verified, totalDebt ≈ 1072 OSMO was added to downPayment ≈ \$40.10 USD, producing a wildly inflated leverage.

Verification

• cargo check and cargo clippy -- -D warnings on backend/ passed clean.
• npx vue-tsc --noEmit and npx eslint on the changed files passed clean.
• npx vitest run src/common/utils/LeaseCalculator.test.ts — all 73 existing tests still pass (no behavioral change in calculateDebt, parseEtlData, etc.; leverageAtOpen is a new additive field).
• Verified the math against the live ETL data for nolus1y6j8c6u8z02f7jfe3wt3nyn7de8cjuk2ak79w5fxatzua8r62zwqufcg25 (the OSMO short used to track this down): LS_cltr_amnt_stable = $40.10, LS_loan_amnt_stable = $29.57, so leverageAtOpen = (40.10 + 29.57) / 40.10 = 1.74 — matches the protocol-true initial leverage and replaces the buggy x12.3 the card previously displayed.

Follow-ups

The dashboard / single-lease detail pages don't show leverage on the card so they're unaffected, but the new leverageAtOpen field is available on LeaseDisplayData if you ever want to surface a frozen-leverage badge elsewhere in the UI.