fix(dashboard): Reclaimable pulse cell opens an interactive picker

[ShogyX]

Summary

Third and final PR for the Dashboard overhaul. Replaces the static "estimated across N fat titles" hint on the Reclaimable pulse cell with a drawer that lets the operator shape the estimate to a real transcode they'd consider running:

• pick target codec / container / bitrate
• filter the source pool by current codec / container / min size / min bitrate
• see candidate count, total current size, projected total size after transcode, and the reclaim — plus the top 20 biggest-reclaim individual files

The actual transcode is queued via the existing optimization-profile flow (the Open Optimization → CTA links there); this endpoint is preview-only per the user's answer to the scope question.

Backend

• New app/services/dashboard/reclaim_preview.py. The reclaim math is intentionally simple:

  est_new_size = duration_seconds * target_bitrate_kbps * 125
  reclaim      = max(0, size - est_new)
  

  Bitrate is the COMBINED video+audio target. Container muxing weight (~1-3%) is ignored — the dashboard preview just needs to be roughly right; the actual transcode is the source of truth.

• New POST /api/v1/dashboard/reclaim-preview endpoint with Pydantic-validated body.

• Files missing duration_seconds or bitrate_kbps drop out (probe failures the transcode couldn't act on anyway).

• Files at or below the target bitrate are excluded (transcoding up would inflate).

Frontend

• New useDashboardReclaimPreview mutation hook (mutate-shape because inputs change on every form interaction).
• Reclaimable pulse cell now opens a Reclaimable storage drawer instead of jumping straight to /optimization.
• New ReclaimPicker component inside the drawer renders the form + headline stats + top-candidate list. Auto-runs the preview on mount.

Test plan

• pytest backend/tests/integration/test_dashboard_reclaim_preview_v111.py — 7 pass (estimate math, at-or-below-target exclusion, source codec / min size filters, files missing probe data drop out, sample ordering, empty-library zero).
• npx tsc --noEmit clean.
• vitest run src/features/dashboard — 40/40 pass.
• Manual on the live install: click Reclaimable, tweak target bitrate / source codec, verify candidates list updates and the per-file reclaim is consistent with operator expectations.

Dashboard overhaul complete

This closes out the three-PR series:

• fix(dashboard): augment Needs-attention card with new attention signals #141 — Needs-attention card augmentation (executables, VT-flagged, junk/unknown, info-tag suggestions, foreign-audio handles unknown).
• fix(dashboard): augment Silently-costing card with transcode breakdowns #142 — Silently-costing card breakdowns (top codecs / devices / reasons / bitrate split).
• fix(dashboard): Reclaimable pulse cell opens an interactive picker #143 — this PR.

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