## Problem
`GetStoreStats` ran a `SUM(CASE WHEN timestamp > ?)` over the full
`observations` table on **every** `/api/stats` call. The staging pprof
analysis (#1460) identified this as rank #9 CPU consumer:
`GetStoreStats.func2` at 920ms cumulative = ~10% of all server CPU.
The query:
```sql
SELECT
COALESCE(SUM(CASE WHEN timestamp > ? THEN 1 ELSE 0 END), 0),
COALESCE(SUM(CASE WHEN timestamp > ? THEN 1 ELSE 0 END), 0)
FROM observations WHERE timestamp > ?
```
scans ~1.9M rows each time `/api/stats` is polled (every 15s from the
dashboard).
## Fix
Add a **30-second TTL cache** on `PacketStore` for `PacketsLastHour` and
`PacketsLast24h`:
- Cache hit → skip the observations goroutine entirely, use stored
values
- Cache miss → run the query, update cache with result
- The node/observer `COUNT(*)` query is unchanged and always runs fresh
The hour/24h counts are display-only values; 30s accuracy is sufficient.
## Changes
`cmd/server/store.go`:
- 4 new fields on `PacketStore`: `statsCacheMu sync.Mutex`,
`statsCacheTime time.Time`, `statsLastHour int`, `statsLast24h int`
- `GetStoreStats`: check cache before launching goroutines; conditional
`wg.Add`; update cache after successful query
Builds clean. No tests changed.
Closes#1460 (P1#1 from staging CPU profile).
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
