feat(defi): DeFi Analytics & Yield Performance Dashboard

[CHEF-SAVY]

closes #386

• Add migration for all analytics snapshot tables (platform, strategy, protocol, AMM, lending, user, reports, export requests)
• Implement platform-wide DeFi summary snapshots with TVL aggregation across savings, AMM, and lending; weighted average yield rate; period-over-period deltas
• Implement strategy performance analytics: effective yield rate, max drawdown, risk-adjusted return, protocol contribution breakdown, benchmark comparison vs Nigerian savings rate (4%)
• Implement yield attribution: per-protocol and per-period breakdown
• Implement protocol analytics: exposure, yield, fee income, IL, health score, uptime, capital efficiency; ranked by yield contribution
• Implement AMM pool analytics: trading volume, fee income, IL, hold-strategy comparison, capital efficiency
• Implement lending portfolio analytics: LTV, health factor, liquidation rate, interest income, unique borrowers
• Implement user-facing analytics: personal savings, yield history, lending position summary
• Implement weekly/monthly/quarterly report generation
• Implement async export requests (platform + user)
• Background worker with configurable interval, schedule-miss alert, and TVL drop alert
• Prometheus gauges: platform TVL, weighted avg yield rate, outstanding loans, avg lending health factor, DeFi revenue
• Prometheus counters: snapshots, cache hits/misses, reports, exports
• Wire all 17 endpoints into main.rs router
• Unit tests: weighted yield rate, risk-adjusted return, AMM capital efficiency, IL vs hold, liquidation rate, protocol efficiency, benchmark delta
• Integration tests: snapshot persistence, lending/strategy/user snapshots, report generation, caching

What was implemented:

• migrations/20270529000000_defi_analytics_dashboard.sql — all analytics snapshot tables (platform,
  strategy, protocol, AMM, lending, user, reports, exports)
• src/defi/analytics/models.rs — all data models and DTOs
• src/defi/analytics/repository.rs — all DB queries (aggregations, inserts, upserts, history)
• src/defi/analytics/service.rs — computation logic (weighted yield rate, risk-adjusted return, AMM
  efficiency, IL vs hold, liquidation rate, benchmark comparison, report generation, exports)
• src/defi/analytics/metrics.rs — Prometheus gauges/counters (TVL, yield rate, loans, health factor,
  revenue, snapshots, cache hits/misses, reports, exports)
• src/defi/analytics/worker.rs — background snapshot job with schedule-miss alert and TVL drop alert
• src/defi/analytics/handlers.rs + routes.rs — all 17 endpoints (admin + user-facing)
• src/defi/mod.rs — added pub mod analytics
• src/lib.rs — exported pub mod defi
• src/main.rs — wired up worker + routes
• tests/defi_analytics_tests.rs — unit tests (weighted yield, risk-adjusted return, AMM efficiency,
  IL vs hold, liquidation rate, protocol efficiency, benchmark delta) + integration tests (snapshot
  insert/retrieve, lending, strategy, user upsert, report generation, caching)

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