Statistics: traffic, drops, devices and connections over time

internal/stats samples every minute what the kernel already counts: the
interfaces' counters (per zone too), Housecarl's table (hits per rule,
built-in and policy; drops by chain; quotas; per-device sets), the
connection tracking table, and housecarld's and the host's memory, CPU
and load. Logged drops feed top lists of sources and what they tried.
Aggregates go to a database of their own (stats.db; per minute 7 days,
per hour 90 days, per day 2 years), with the store and queries from
Jarl's.

Traffic per inside device is a setting (stats_per_device): counting sets
stats_up/down_v4/v6 and rules at the top of the forward chain, IPv6 per
/64, carried across reloads. Set names starting with meter_, conn_,
quota_ or stats_ are reserved (set_name_reserved).

GET /stats/{status,summary,timeline,top,hits,heatmap}, hc stats, a
Statistics page with Jarl's charts, two charts on the dashboard, and a
conntrack_full warning. e2e: 188 checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/cli.md

@@ -138,6 +138,7 @@ by name.
 | `quotas` | what each rule's quota used (each address's, for per-address quotas), and when it resets |
 | `log [-f] [--search TEXT] [--limit N]` | the packets logged by rules and policies ([operations.md](operations.md#the-log)) |
 | `connections [--search TEXT] [--limit N]`, `connections close --search TEXT [-y]` | tracked connections; closing makes the rules check them again ([operations.md](operations.md#connections)) |
+| `stats [--since 24h]` | traffic per zone, drops, the busiest rules, blocked sources, devices; `stats status`, `stats traffic`, `stats top devices\|blocked\|tried\|quota ID`, `stats rules [--unused]`, `stats connections` ([stats.md](stats.md#the-cli)) |
 | `matrix [FROM TO]` | what the rules do to new connections between zones; with FROM and TO, that pair's rules in order ([rules.md](rules.md#the-zone-matrix)) |
 | `trace [--address A] [--port N] [--protocol P] [--seconds S]` | follow packets through every table's rules ([operations.md](operations.md#tracing)) |
 
@@ -207,7 +208,7 @@ See [nat.md](nat.md).
 | Command | |
 |---|---|
 | `settings show` | |
-| `settings set [--input-policy\|--forward-policy\|--output-policy accept\|drop] [--log-drops on\|off] [--builtin NAME=on\|off]... [--own-prefix P]... [--antispoof-v4 P]... [--antispoof-v6 P]... [--flowtable DEV]... [--probation 2m] [--auto-confirm on\|off]` | lists given replace the old ones |
+| `settings set [--input-policy\|--forward-policy\|--output-policy accept\|drop] [--log-drops on\|off] [--builtin NAME=on\|off]... [--own-prefix P]... [--antispoof-v4 P]... [--antispoof-v6 P]... [--flowtable DEV]... [--probation 2m] [--auto-confirm on\|off] [--stats-per-device on\|off]` | lists given replace the old ones |
 
 ### probation, history, import