Async vision: decouple run_vision from save path

[geirolafs]

Task

Per #8 decision. Part of map #1.

run_vision is called synchronously inside process_and_save (lib.rs) — every save (drag-drop, paste, extension, clipboard watch) blocks ~5.9 s on the keep-vision spawn (#7: spawn is 40× the rest of the 148 ms pipeline). Bulk import at 20k extrapolates to 34 h.

Fix

• Remove run_vision from the save pipeline — save returns ~150 ms; card appears instantly with palette + thumbhash
• Backfill vision_tags + ocr_text async afterward via the existing analyze_vision_item path
• Emit/refresh so tags appear in UI when backfill lands

Deferred (not this ticket)

Persistent keep-vision sidecar / batch mode to amortize per-spawn model load — only if backfill throughput (~0.17 img/s) actually hurts.