Fix stats query

[j-chmielewski]

Related issues: #1071, #1062, #874, #1062

Fixes excessive disk writes caused by an inefficient stats query:

• Adds upload_diff / download_diff columns to location_stats and tunnel_stats, computed once at insert time
• Rewrites the stats queries to read those precomputed diffs, replacing the window-function CTE with a plain GROUP BY aggregate - no more large SQLite temp files
• Drops stats retention from 30 days to 28 hours, since the UI only ever charts the last 24h
• Adds covering indexes on (location_id, collected_at) and (tunnel_id, collected_at) so the aggregate queries resolve via an index search instead of a full table scan