Add caching strategy documentation for API routes

[rogerSuperBuilderAlpha]

Path: lib/rate-limit-server.ts, app/api/*/route.ts
Size: S

Problem / Context: API routes have comment about revalidateTag() (line 465 of signup route) but caching strategy is scattered. Developers don't know cache TTLs.

Proposed scope:

• Create docs/API_CACHING.md
• Document: which endpoints are cached, TTL, revalidation strategy
• Explain revalidateTag usage (hack-a-sprint-2026 showcase, event signup)
• Link to Next.js caching guide

Acceptance criteria:

• Caching strategy documented for ≥10 API routes
• TTLs explicit (e.g., 30s cache + revalidateTag on mutation)
• Examples provided

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[mrrCarter]

Opened PR for this: #1356

[github-actions]

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