fix: check status code before decoding getJSON responses (!strims)

[nom-d-plume]

Problem

!strims/!stream has been returning error getting api data in chat.

Root cause

This was originally an unclosed resp.Body leak in getStreamList,
which #65 (Modernize) already fixed as part of the getJSON refactor.
Rebased onto that; the remaining gap is that getJSON (backing
getStreamList/!strims and getProfileInfo) still decodes the
response body regardless of resp.StatusCode. A non-200 response
(5xx, rate limiting, an edge/WAF error page, etc.) gets fed straight
into json.Decoder, producing either a confusing decode error or a
silently empty result if the error body happens to parse as valid
JSON of a different shape.

Fix

Check resp.StatusCode in getJSON and fail fast with a descriptive
error before decoding.

Verification

• go build ./..., go vet ./..., go test ./... all pass.
• Manual repro: called printTopStreams with logOnly=true against the
  live https://strims.gg/api endpoint on top of current master;
  confirmed it returns the top 3 streams correctly, e.g.:

  13 strims.gg/angelthump/nomdeplume [nsfw]
  6 strims.gg/angelthump/gwsd7259
  6 strims.gg/angelthump/thekekinator
  

[nom-d-plume]

cc @jbpratt @xDashh @slugalisk @real-salad @tensei @HoppenR @SoMuchForSubtlety @talleyp @x-Xymos — could one of you take a look? I don't have permissions to formally request reviewers on this repo.

[nom-d-plume]

Rebased onto master (#65 already fixed the resp.Body leak via the getJSON refactor — thanks for that). Retargeted this to the remaining gap: getJSON decoding non-200 responses without checking status code. No conflicts now.