feat([issue-5855]): sweep abandoned resumable-download leftovers

[atomanticagent]

Summary

• Add sweepOrphanedPartials so leftover ${dest}.partial files (and their .partial.etag sidecars) older than 7 days are removed from the dirs the resumable-download path can write to: LoRAs, spec-decode GGUF parents, Ollama, and LM Studio.
• Wire a delayed boot pass plus a daily tick in bootstrap.js (same createSweepScheduler shape as the image-refs / image-clean GCs). Recent files and in-flight spec-decode dests are never touched.
• Focused unit tests cover old-removed, recent-kept, in-flight-protected, missing-dir, nested trees, and sidecar cleanup.

Closes #5855

Test plan

• cd server && npx vitest run lib/downloadPreflight.test.js — 32 passed
• Boot a running install, confirm orphaned-partial-gc is scheduled (no deletion of a fresh .partial)
• Place a 8-day-old .partial (+ .etag) under data/loras/ and confirm the next sweep removes both without touching neighboring complete files