shared hashes: separate background job with visible state

- compute duplicate-hash statistics in their own job (startup, hourly,
  on demand) so they no longer block the table stats
- single-pass query over DISTINCT (hash, addon_id), 15 min timeout
- record errors and panics; never stay stuck in a computing state
- whitelist page polls while computing, shows errors and a Recompute
  button; tab is linkable via ?tab=shared

docs/admin-dashboard.md

@@ -20,7 +20,7 @@ change data must also send `X-Admin-CSRF: 1`.
 
 | Area | Endpoints |
 |---|---|
-| Stats | `GET overview`, `GET overview/duplicates`, `GET traffic?from&to`, `GET top?kind=addon\|search&days` |
+| Stats | `GET overview`, `GET overview/duplicates` (job state + top shared hashes), `POST overview/duplicates/refresh`, `GET traffic?from&to`, `GET top?kind=addon\|search&days` |
 | Queue | `GET queue`, `POST queue/reset-stuck` |
 | Addons | `GET addons?q&status&type&sort&order&limit&offset`, `GET addons/:id`, `GET addons/:id/files` |
 | Addon actions | `POST addons/:id/{priority\|to-be-indexed\|reindex\|reset-indexing\|recheck\|restore}` with body `{"value": bool}`, `DELETE addons/:id`, `DELETE addons/:id/purge?confirm=<id>`, `POST bulk/addons` |