feat: add differentiated Cache-Control headers to stream list responses

[devEunicee]

Summary

Sets differentiated Cache-Control headers on GET /api/streams and GET /api/streams/:id based on the mutability of the returned stream data.

Changes

• src/streams/status.ts — add isTerminalStatus(status: ApiStreamStatus): boolean helper
• src/routes/streams.ts — derive and set Cache-Control before res.json in both GET handlers
• tests/routes/streams.cacheControl.test.ts — 12 tests (all passing): active/paused/scheduled → no-store, completed/cancelled → cacheable, mixed page → no-store, empty page → cacheable
• docs/api.md — document caching behaviour per stream status

Cache-Control logic

Condition	Header
Any stream is active, paused, or scheduled	private, no-store
All streams are completed or cancelled	public, max-age=300, stale-while-revalidate=60
Empty page	public, max-age=300, stale-while-revalidate=60

Terminal streams are immutable — safe to cache at the edge. Non-terminal streams carry live financial state that changes continuously and must not be stored by shared caches.

Test results

Closes #234

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