next -> main (1.0.0 milestone) (#105)

Accumulating milestone branch. One squashed commit per closed issue; `next` is kept green and mergeable to `main` at any time without notice.

Landed so far:

- `chore: update golangci-lint to v2.12.2 with canonical config` (#54) — canonical v2-schema `.golangci.yml`, pins bumped in `Dockerfile` and `script/bootstrap`, tree at `0 issues.`. Three behaviour deltas are recorded in that PR's body: `Cache.StoreVariant` takes a context, `MetadataStorage.Store` no longer leaks temp files on failure, and the `signing_key` too-short error text gained a `value too short:` prefix.

Sequencing for the milestone is tracked in #103.

Reviewed-on: #105
Co-authored-by: clawbot <clawbot@noreply.example.org>

internal/imgcache/imgcache.go

@@ -90,6 +90,7 @@ func (r *ImageRequest) SourceURL() string {
 	if r.AllowHTTP {
 		scheme = "http"
 	}
+
 	url := scheme + "://" + r.SourceHost + r.SourcePath
 	if r.SourceQuery != "" {
 		url += "?" + r.SourceQuery