Fix Alpha Vantage rate-limit failures in the daily alerts worker

[valuecodes]

What

• The daily alerts worker was failing with Alpha Vantage's 1-request-per-second rate-limit error. Both SP500 alerts run on the same 08:00 cron and each built its own client, so four identical requests (2 alerts × daily+weekly) fired at once and tripped the free-tier burst limit.
• scheduled.ts now builds one AlphaVantageClient per run and passes it to every alert via AlertContext.
• AlphaVantageClient de-duplicates concurrent in-flight requests, collapsing those four calls into two. Entries are dropped once a request settles, so a reused client still fetches fresh data and a failure never poisons later calls.

How to test

pnpm typecheck
pnpm lint
pnpm test
pnpm build

Security review

No security-impacting changes.

[Copilot]

Pull request overview

This PR reduces Alpha Vantage free-tier burst rate-limit failures in the apps/alerts scheduled worker by sharing a single AlphaVantageClient per cron run and de-duplicating concurrent identical requests within that client.

Changes:

• Add in-flight request de-duplication to AlphaVantageClient (no persistent caching; evicts on settle).
• Construct one AlphaVantageClient per scheduled run and pass it to alerts via AlertContext.
• Update/extend tests to cover de-duplication behavior and verify the scheduled handler shares one client instance.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/alpha-vantage/src/alpha-vantage.ts	Add in-flight request de-duplication and shared error handling helper.
packages/alpha-vantage/src/alpha-vantage.test.ts	Add unit tests verifying concurrent de-duplication and eviction behavior.
apps/alerts/src/scheduled.ts	Build a single Alpha Vantage client per run and pass it into alerts.
apps/alerts/src/scheduled.test.ts	Assert the scheduled handler constructs exactly one Alpha Vantage client per run.
apps/alerts/src/alerts/types.ts	Extend AlertContext to include a shared alphaVantage client.
apps/alerts/src/alerts/sp500-drawdown.ts	Use shared alphaVantage client from context instead of constructing per-alert.
apps/alerts/src/alerts/sp500-drawdown.test.ts	Update test harness to provide alphaVantage via context.
apps/alerts/src/alerts/sp500-close.ts	Use shared alphaVantage client from context instead of constructing per-alert.
apps/alerts/src/alerts/sp500-close.test.ts	Update test harness to provide alphaVantage via context.

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