Comprehensively adds { timeout: 120_000 } to every HEIF/HEIC
integration test across 40 test files. HEIF decoding on CI runners
is significantly slower than local, causing sporadic timeouts at
the default 5s or previous 60s limits.
The upscale route held the HTTP connection open for the full duration of
Python sidecar processing (30-300s). Behind proxies with connection
timeouts (Cloudflare Tunnel: 100s), this caused HTTP 524 errors.
The route now returns 202 Accepted immediately after upload validation
and processes in the background. The result (downloadUrl, sizes, etc.)
is delivered via the existing SSE progress channel. The frontend detects
the 202 and waits for the SSE completion event instead of reading the
XHR response body. A reconnect-safe completion store ensures results
survive brief SSE disconnects.
Closes#106