Fix segfault: keep JSON body alive for parsed string refs

- Root cause: parseFromSlice returns slices pointing into the raw JSON
  body, but body was freed immediately after parsing in getUpdates
- Fix: return OwnedParsed wrapper that keeps both parsed result and
  raw body alive together
- Switch HTTP layer to curl subprocess (more reliable than std.http
  with static linking)
- Fix downloadToFile: use spawnAndWait instead of collectOutput
  (collectOutput requires both pipes)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,4 +1,4 @@
 FROM alpine:3.21
-RUN apk add --no-cache ffmpeg ca-certificates
+RUN apk add --no-cache ffmpeg curl ca-certificates
 COPY zig-out/bin/transcribator /usr/local/bin/
 CMD ["transcribator"]