feat: cache soroban contracts, decouple notifications, tenant isolation, read replica

[Ogstevyn]

Summary

• Adds a TTL-based in-memory cache (ContractCache) in fluid-server for Soroban contract definitions and ledger footprint schemas, with a background eviction task and HTTP endpoints (/contract/definition, /contract/footprint, /contract/cache/stats). TTL is configurable via CONTRACT_CACHE_TTL_SECS.
• Decouples the fluid-server alert delivery path into a standalone NotificationEngine Tokio background task. The hot request path enqueues events on a bounded channel (NotificationHandle::send); backends (Slack webhook, extensible to SMTP/PagerDuty) consume events asynchronously with no impact on fee-bump latency.
• Adds a tenantIsolationMiddleware and requireTenantId helper in server that extract a verified tenantId from the resolved API key and gate all downstream database access to that tenant. A tenantIsolationService provides scoped query helpers (getTenantTransactions, getTenantApiKeys, getTenantWebhookDeliveries, getTenantDailySpend, getTenantById, getTenantQuotaTopUps) that hard-code the tenantId predicate.
• Introduces a dedicated readPrisma client (server/src/utils/readDb.ts) backed by READ_REPLICA_URL (falls back to DATABASE_URL). Analytics queries in adminAnalytics.ts are migrated to this client so analytical reads no longer contend with write transactions on the primary database.

Test plan

• fluid-server: 8 unit tests in contract_cache.rs (#[tokio::test]) covering set/get, TTL expiry, invalidation, eviction, overwrite, and count. 7 unit tests in notifications/mod.rs covering severity labels, serialisation, channel capacity, handle cloning, multi-event processing, and backend name routing.
• server: Unit tests in tenantIsolationService.test.ts (14 cases), tenantIsolation.test.ts (7 cases), readDb.test.ts (5 cases). Existing adminAnalytics.test.ts updated to mock readDb instead of db to reflect the replica switch.
• Documentation added under docs/ for all four features.

closes #706
closes #708
closes #702
closes #703

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