refactor(I5): remove one-and-done GS/IR migration tooling

The GS/IR migration cutover is complete, so the runbook tooling is dead
weight. Removed:
- services/migrators/ (gs_ingest, ir_ingest, tag_apply, ml_queue, verify,
  cleanup), tasks/migration.py, api/migrate.py (+ blueprint registration)
- MigrationRun model; alembic 0027 drops the migration_run table
- frontend LegacyMigrationCard + migration store (+ MaintenancePanel ref)
- celery include + task route + celery_signals queue mapping for migration.*
- the 1 GB MAX_CONTENT_LENGTH / MAX_FORM_MEMORY override (added solely for
  the ir_ingest upload)
- migration-surface tests (test_api_migrate, test_migration_verify,
  test_ir_ingest, test_gs_ingest, test_tag_apply)

Kept: the alembic schema-migration tests (test_migration_00XX — unrelated)
and cleanup_service.py (the permanent artist-cascade/unlink home).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/migration.js

@@ -1,83 +0,0 @@
-import { defineStore } from 'pinia'
-import { ref, computed } from 'vue'
-import { useApi } from '../composables/useApi.js'
-
-const POLL_MS = 2000
-
-export const useMigrationStore = defineStore('migration', () => {
-  const api = useApi()
-
-  const activeRun = ref(null)
-  const recentRuns = ref([])
-  const error = ref(null)
-  const loading = ref(false)
-
-  let pollHandle = null
-
-  async function trigger(kind, params, file = null) {
-    loading.value = true
-    error.value = null
-    try {
-      let body
-      if (file) {
-        // Multipart upload for ingest kinds.
-        const form = new FormData()
-        form.append('export_file', file)
-        if (params && params.dry_run) form.append('dry_run', 'true')
-        if (params && params.force) form.append('force', 'true')
-        body = await api.post(`/api/migrate/${kind}`, { body: form })
-      } else {
-        body = await api.post(`/api/migrate/${kind}`, { body: params || {} })
-      }
-      await loadRun(body.run_id)
-      pollActive(body.run_id)
-      return body
-    } catch (e) {
-      error.value = e
-      throw e
-    } finally {
-      loading.value = false
-    }
-  }
-
-  async function loadRun(runId) {
-    activeRun.value = await api.get(`/api/migrate/runs/${runId}`)
-  }
-
-  async function loadRecent() {
-    recentRuns.value = await api.get('/api/migrate/runs', { params: { limit: 10 } })
-  }
-
-  function pollActive(runId) {
-    stopPolling()
-    pollHandle = setInterval(async () => {
-      try {
-        const run = await api.get(`/api/migrate/runs/${runId}`)
-        activeRun.value = run
-        if (run.status === 'ok' || run.status === 'error') {
-          stopPolling()
-          await loadRecent()
-        }
-      } catch (e) {
-        error.value = e
-        stopPolling()
-      }
-    }, POLL_MS)
-  }
-
-  function stopPolling() {
-    if (pollHandle) {
-      clearInterval(pollHandle)
-      pollHandle = null
-    }
-  }
-
-  const isRunning = computed(
-    () => activeRun.value && activeRun.value.status === 'running',
-  )
-
-  return {
-    activeRun, recentRuns, error, loading, isRunning,
-    trigger, loadRun, loadRecent, pollActive, stopPolling,
-  }
-})