Serve full-resolution PNG output (fix 'Open full' giving a 512px thumbnail)

[bscholer]

Summary

The "Open full" affordance on the terminal node linked to `/api/preview`, which thumbnails every artifact (PNG and FITS alike) to `THUMB_LONG_EDGE = 512` px. The `save_image` node writes a full-resolution PNG via Siril's `savepng`, but the UI never had a way to fetch it — users opened the link and got a tiny version of their multi-megapixel export.

Shape

• New `/api/output/{node_hash}/{port}` endpoint:
  • PNG outputs: the committed file is returned as-is (no re-encode, no thumbnail).
  • FITS outputs: autostretched at full resolution, cached inside the entry as `output.png`. Repeat opens are a file read.
• `server/preview.py` refactored to share dispatch logic; each renderer now takes a `max_dim: int | None` (None = full res). Cache filenames namespaced as `preview.png` vs `output.png` so thumbnail and full-res renders coexist in one entry.
• UI `api.outputUrl()` mirrors `previewUrl` / `previewUrlStretched`. `PipelineRow`'s "Open full" button uses it. Tile thumbnails still call `previewUrl()`.

Test plan

• 4 new tests in `tests/test_render_output.py` covering PNG pass-through (returns original file), FITS full-res (no `.thumbnail()` clipping), output caching (idempotent file path), preview/output cache namespacing.
• Existing preview tests still pass (34/34).
• Full suite: 682 passed, 1 skipped.
• Ruff clean; svelte-check 0 errors.
• In a browser: complete a project, click "Open full" on the final node, verify the PNG that opens is the full `save_image` resolution rather than a 512px thumbnail.