Deduplicate (hostname, metric_id) rows across verticals using DISTINCT ON in
GET /items, GET /items/:hostname, GET /vcl/stats (heavy-hitters + forecast),
GET /mttr, and persistUpload() snapshot block. Add defensive groupByHostname
Set and hostname_status CTE for snapshot classification.
Includes 38 property-based tests (11 exploration + 27 preservation) covering
all six affected sites.
Closes#13
