fix: namespace automatic downloads by project id

[runshouse]

Prefix automatic SDK download filenames with the returned project ID so separate projects whose output URLs share a basename do not overwrite one another. All eight image/video/audio/face-detection sync/async callers pass that ID through the existing downloader; direct helper calls without it retain their previous naming. README calls out the filename change and downloaded_paths as the authoritative saved paths.

Six files, 23 insertions / 3 deletions. Independent exact-source review passed. On devbox, all 169 existing Python3.12 tests passed in 35.22 seconds; mypy over 241 source files, existing parameter-sync and whitespace checks passed. Direct execution of the existing path helper returned ./project-a-0.mp4 and ./project-b-0.mp4 for two projects sharing the URL basename; omitting the optional ID retained ./0.mp4. These were path computations, not new download fixtures or live media transfers; existing suites do not cover cross-project collisions. No tests, fixtures, dependencies, HTTP options, directory creation policy, polling cadence or error semantics changed. Same-project re-download and duplicate basenames within a project can still overwrite; this is not universal collision prevention. Main-based and independently shippable, not stacked on polling-options PR183.