feat(stats): 24-hour hourly traffic histogram with duty-cycle estimate

[iceice400]

Summary

• Add GET /api/stats/hourly?hours=24 — SQL-backed hourly Meshtastic/MeshCore packet counts with estimated time-on-air and duty-cycle %
• New src/analytics/toa_estimate.py helper (shared with future PR 7 duty-cycle work)
• Stats tab: Traffic (24h) stacked bar chart + duty-cycle line; EU868 shows dashed 1% reference limit

Why

The existing 60-minute timeline scans only the last 2000 in-memory packets. Backbone nodes need accurate 24h history from SQLite (idx_packets_timestamp).

Type

• Bug fix
• Feature
• Docs
• Refactor
• UI
• Installer
• Region support
• Hardware change

Testing

• Local unit tests
• Tested on hardware
• Dashboard tested on device

python -m unittest tests.test_traffic_hourly -v

Manual checklist (on gateway):

1. Stats tab → Traffic (24h) chart appears below 60-min chart
2. Stacked bars show Meshtastic + MeshCore per hour; empty hours show zero height
3. Duty cycle line labelled (est.) on secondary axis
4. radio.region: EU_868 → dashed 1% limit line; US → no limit line
5. Leave Stats tab open 5+ min → chart updates without flicker (data patch, not recreate)
6. Compare one hour bucket to manual SQL on packets table

Hardware: RAK7248 or Pi + HAT · US915 / EU868

Impact

• Parsing
• Relay
• TX
• Radio driver
• Region logic
• UI + read-only API
• Installer only

Risks: Low — read-only SQL aggregation; ToA is an estimate only (does not drive TX throttle). No schema migration.

AI-assisted?

• Yes