feat(startup): hot startup — load hotStartupHours synchronously, fill retentionHours in background

[efiten]

Closes #1183

Summary

• Adds packetStore.hotStartupHours config key (float64, default 0 = disabled). When set, Load() loads only that many hours of data synchronously, cutting cold startup from 5–20 minutes to under 60 seconds on large DBs.
• A background goroutine (loadBackgroundChunks) fills the remaining retentionHours window in daily chunks after startup completes. Analytics indexes are rebuilt once at the end.
• QueryPackets and QueryGroupedPackets check oldestLoaded and fall back to db.QueryPackets() for any query whose Since/Until predates the in-memory window — covering days 8–30 permanently (beyond retentionHours) and the background-fill gap during startup.
• /api/perf/store gains hotStartupHours, backgroundLoadComplete, and backgroundLoadProgress fields so operators can monitor the fill.

Configuration

"packetStore": {
  "retentionHours": 168,
  "hotStartupHours": 24
}

hotStartupHours: 0 (default) preserves existing behavior exactly.

Test plan

• TestHotStartupConfig_Clamp — clamping when hotStartupHours > retentionHours
• TestHotStartupConfig_ZeroIsDisabled — zero leaves feature disabled
• TestHotStartup_LoadsOnlyHotWindow — only hot-window packets in memory after Load()
• TestHotStartup_DisabledWhenZero — all retention packets loaded when disabled
• TestHotStartup_loadChunk_AddsOlderData — chunk merges correctly, ASC order maintained
• TestHotStartup_BackgroundFillsToRetention — background goroutine fills to retentionHours
• TestHotStartup_ChunkErrorRecovery — chunk SQL failure logged and skipped, loop terminates
• TestHotStartup_SQLFallback_TriggeredForOldDate — query before oldestLoaded routes to SQL
• TestHotStartup_SQLFallback_NotTriggeredForRecentDate — recent query stays in-memory
• TestHotStartup_PerfStats — new fields present in /api/perf/store response

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