feat: File placement card — survey, plan per artist, apply, put back (4246, slice 3c)

The UI half of the reconciler, in Maintenance. Survey shows how many images
sit in the wrong artist's folder and which folders they are in; each artist
gets its own Plan button; each run can be reviewed, applied, and put back.

Deliberately NOT using useMaintenanceTask. That composable stashes a task id
in localStorage so a result survives navigate-away, which is the right answer
when the only record is a Celery result. Here the runs are database rows — so
a reload, another machine, or coming back tomorrow simply shows the same
state, because the state IS the row. The card polls the runs endpoint instead.

Copy avoids the vocabulary this work has been tripping over: "folder", "put
back", "in the wrong folder" rather than artist_id, revert and canonical. The
one thing the operator most needs to know — nothing here changes who an image
belongs to — is what the blurb says first.

Three things I had assumed and checked instead: MaintenanceTile lives in
common/ not settings/; there is no generic ConfirmDialog (BackupCard uses a
purpose-built modal), so this uses a plain v-dialog; and `loadArtistNames`
did not exist — it does now, mapping id to name so a run row reads "Conto"
rather than "#47". The name deliberately is not denormalised into the run: it
belongs to the artist and would go stale on a rename.

Also extracted `stubFetch` to frontend/test/stubFetch.js. The shape ledger
flagged it as a byte-identical duplicate across five specs and this would
have been the sixth; the others keep their copies until each is next touched.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/src/stores/cleanup.js

@@ -71,10 +71,66 @@ export const useCleanupStore = defineStore('cleanup', () => {
     return await api.post(`/api/cleanup/audit/${id}/cancel`)
   }
 
+  // --- placement reconciler (milestone #421) --------------------------------
+  //
+  // Runs are server-side rows, so the DATABASE is the durable state here —
+  // no localStorage resurfacing (useMaintenanceTask) is needed. Reload the
+  // page, open it on another machine, and the run and its status are simply
+  // there. That also means a plan survives being walked away from for a day.
+
+  const placementRuns = ref([])
+  const layout = ref(null)
+
+  // The survey: which rows sit outside their artist's directory. check_disk
+  // additionally stats every destination (collisions, missing sources) and
+  // costs one stat per misplaced row over NFS, so it is opt-in.
+  async function loadLayout(checkDisk = false) {
+    layout.value = await api.get('/api/cleanup/layout', {
+      params: checkDisk ? { check_disk: 1 } : {},
+    })
+    return layout.value
+  }
+
+  // id -> name, so a run row can say "Conto" instead of "#47". The runs
+  // endpoint carries artist_id alone: the name belongs to the artist, and
+  // denormalising it into every run would go stale the moment one is renamed.
+  async function loadArtistNames() {
+    const rows = await api.get('/api/artists/names')
+    return Object.fromEntries((rows || []).map(a => [a.id, a.name]))
+  }
+
+  async function loadPlacementRuns(limit = 25) {
+    const body = await api.get('/api/cleanup/placement/runs', { params: { limit } })
+    placementRuns.value = body.runs || []
+    return placementRuns.value
+  }
+
+  // Detail carries `moves` — the plan the operator reads before agreeing.
+  async function getPlacementRun(id) {
+    return await api.get(`/api/cleanup/placement/runs/${id}`)
+  }
+
+  async function planPlacement(artistId = null) {
+    return await api.post('/api/cleanup/placement/plan', {
+      body: artistId === null ? {} : { artist_id: artistId },
+    })
+  }
+
+  async function applyPlacement(id) {
+    return await api.post(`/api/cleanup/placement/runs/${id}/apply`)
+  }
+
+  async function revertPlacement(id) {
+    return await api.post(`/api/cleanup/placement/runs/${id}/revert`)
+  }
+
   return {
     defaults, recentRuns,
     loadDefaults,
     previewMinDim, deleteMinDim,
     startAudit, getAudit, loadHistory, latestAuditForRule, applyAudit, cancelAudit,
+    placementRuns, layout,
+    loadLayout, loadArtistNames, loadPlacementRuns, getPlacementRun,
+    planPlacement, applyPlacement, revertPlacement,
   }
 })