Fix Apollo Server 5 Express integration and harden finance cluster service

[Copilot]

The finance cluster integration (OpenTrading / Portfolio-Watcher / tax-break behind a unified GraphQL API) already exists on main, but the service could not actually start: @apollo/server is pinned to ^5.5.1 and Apollo Server 5 removed the @apollo/server/express4 subpath, so npm start died at import time.

Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './express4' is not defined
by "exports" in node_modules/@apollo/server/package.json imported from src/index.js

Changes

• Server bootstrap (src/index.js): import expressMiddleware from @as-integrations/express4 (peer range @apollo/server ^4 || ^5, express ^4) instead of the removed built-in subpath. No API or context wiring changes.
• Batching (src/services/financeService.js): the TTL cache now tracks in-flight promises, so concurrent resolvers hitting the same upstream key (trading, portfolio) share one connector call rather than fanning out duplicates. Cache expiry is stamped on resolution rather than on request start.
• Tests (test/finance.test.js): unit coverage for the layers previously only exercised end-to-end — field normalization across inconsistent upstream shapes, tradeToTaxEvent (sell vs. buy), portfolio aggregation totals, trade filtering, year-scoped tax estimation, env-driven config defaults/overrides, and the new in-flight dedup.
• README: stack line corrected to Apollo Server 5 + Express 4 via @as-integrations/express4; data-flow section documents the caching/de-duplication step.

Notes for reviewers

• The dependency addition is the minimum needed to keep Apollo Server 5 with Express 4. The alternative — migrating to Express 5 + @as-integrations/express5 — is a larger change and was left out.
• Follow-ups unchanged from the original integration: live authenticated connectors, pagination/date filters on trades and snapshots, persistent cache if upstream latency becomes material.

• Fixes Implement Priority 1 Finance Cluster Integration via Unified GraphQL API #13

[Copilot]

🟡 Changes recommended

The newly added @as-integrations/express4@1.1.2 requires Node >=20 (per lockfile) while the repo still advertises Node >=18 in package.json/README, creating an install/runtime support mismatch that should be resolved.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR fixes the service startup break caused by Apollo Server 5 removing the @apollo/server/express4 export, and strengthens the finance cluster service by adding in-flight request de-duplication plus targeted unit tests and documentation updates.

Changes:

• Switch Express integration to expressMiddleware from @as-integrations/express4 so npm start works with Apollo Server 5.
• Add an in-flight promise map to the finance TTL cache to dedupe concurrent upstream reads and stamp expiry on resolution.
• Add finance unit tests (normalization, aggregation, tax estimation, config, and batching) and update README to document the stack and caching behavior.

File summaries

File	Description
src/index.js	Updates Apollo/Express middleware import path for Apollo Server 5 compatibility.
src/services/financeService.js	Adds in-flight de-duplication around the existing TTL cache to reduce duplicate upstream calls.
test/finance.test.js	Introduces unit tests covering finance domain normalization, aggregation, config, and caching/batching behavior.
README.md	Updates the stack description and documents caching/de-duplication in the finance data-flow.
package.json	Adds @as-integrations/express4 dependency required for Apollo Server 5 + Express 4 integration.
package-lock.json	Locks the new dependency and its metadata (including Node engine requirement).

Review details

• Files reviewed: 5/6 changed files
• Comments generated: 3
• Review effort level: Lite

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