test: add unit and integration tests for alerts, airdrops, and webhooks

[chinoyenmose]

Fixes #158
Fixes #159
Fixes #160

What changed

• No unit tests for alerts service #158: Added alerts tests for isTriggered edge cases (boundary values, unknown type), create normalization (uppercase asset, default repeat, unique IDs), fire payload verification, and multiple alerts per asset
• No unit tests for airdrop service #159: Added airdrop tests for cancel preserving recipients, addRecipients with empty input, listRecipients pagination, contract_airdrop_id handling, and cancel on completed airdrop
• No integration tests for webhook CRUD API #160: Added webhook integration tests for API contract (secret not leaked in GET), default active state, wildcard/multiple event subscriptions, PATCH events and URL updates, and empty deliveries endpoint

Why

• Alert logic determines when price thresholds are hit — bugs could cause missed or false alerts
• Airdrop service is core business logic — bugs could cause incorrect token distributions
• Webhook management is user-facing — API contract bugs affect integrators

How to test

• Run npx jest test/alerts.test.js test/airdrops-service.test.js test/webhooks.routes.test.js
• All 103 tests pass across the 3 modified test files

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