feat: Add pending exposure tracking, mm_ratio, and new risk engine commands

[shanu516516]

Summary

• Pending exposure tracking: Added total_pending_long_btc and total_pending_short_btc to the risk_engine_update table, Diesel schema, and MarketRiskStatsResponse. The archiver now persists pending exposure from RiskState on every risk engine event.
• New risk engine commands: Handle AddPendingExposure, RemovePendingExposure, and RecalculateExposure variants from relayer-core. Each is archived with its position type and amount.
• mm_ratio risk parameter: Added mm_ratio (maintenance margin ratio) to RiskParams, the risk_params_update table, and the archiver's param-update handler. Defaults to 0.4 via RISK_MM_RATIO env var and #[serde(default)].

Database Migrations

Migration	Change
2026-02-21-000000_risk_engine_pending_exposure	ALTER TABLE risk_engine_update ADD COLUMN total_pending_long_btc, total_pending_short_btc
2026-02-21-100000_add_mm_ratio_to_risk_params	ALTER TABLE risk_params_update ADD COLUMN mm_ratio

Files Changed

• src/archiver.rs — match arms for 3 new RiskEngineCommand variants; populate pending exposure and mm_ratio fields
• src/database/models.rs — total_pending_long_btc, total_pending_short_btc on RiskEngineUpdateRow / NewRiskEngineUpdate; mm_ratio on RiskParamsUpdateRow / NewRiskParamsUpdate
• src/database/schema.rs — Diesel schema updated for both tables
• src/rpc/types.rs — mm_ratio added to RiskParams with serde default; pending fields added to MarketRiskStatsResponse
• src/rpc/util.rs — compute_market_risk_stats now includes pending exposure in response
• Cargo.toml — relayer-core dependency updated to bug-indentify branch

Test plan

• cargo check passes — verified
• Run pending migrations on dev database
• Verify get_market_stats returns total_pending_long_btc and total_pending_short_btc fields
• Send AddPendingExposure / RemovePendingExposure / RecalculateExposure events through archiver — rows inserted correctly
• Verify mm_ratio is persisted in risk_params_update on a RiskParamsUpdate event