feat/streams conditional get

[MerlinTheWhiz]

PR Summary

Closes #484

Description

GET /api/streams/:id already computed and set an ETag header (streamEntityTag(metadata)), but never read the inbound If-None-Match request header. This meant well-behaved HTTP caches and polling clients always received a full 200 body even when their cached representation was still valid.

This PR implements conditional GET support per RFC 7232 §3.2, allowing clients to skip re-downloading unchanged stream bodies.

Changes

File	Change
src/routes/streams.ts	Added matchesIfNoneMatch() helper (weak comparison per RFC 7232, supporting *, comma-separated lists, and W/ prefix). Modified GET /:id handler to check If-None-Match after computing the ETag and return 304 Not Modified with no body on match.
src/openapi/spec.ts	Documented the 304 response with ETag and Last-Modified headers, plus the If-None-Match request header on GET /api/streams/{id}.
tests/routes/streams.test.ts	8 new tests covering: exact ETag match, * wildcard, comma-separated lists, non-matching tag, absent header, 304 header presence, empty body on 304, and 200 envelope shape preservation.

Acceptance criteria

• Matching If-None-Match returns 304 with no body and the ETag header
• Non-matching / absent If-None-Match returns the existing 200 envelope
• * and comma-separated tag lists are handled
• Weak comparison (RFC 7232) — W/ prefix is stripped before comparing
• OpenAPI documents the 304 response

Security notes

The ETag is derived from server-side metadata only (id + updated_at), using a SHA-256 hash. No client-controlled input flows into the tag. The 304 response leaks no body content.

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