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

admin/routes.go

@@ -21,6 +21,7 @@ func Register(r *gin.Engine) {
 
 	p.GET("/overview", overview)
 	p.GET("/overview/duplicates", duplicateHashes)
+	p.POST("/overview/duplicates/refresh", refreshDuplicates)
 
 	p.GET("/queue", queue)
 	p.POST("/queue/reset-stuck", resetStuck)