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>

CHANGELOG.md

@@ -70,6 +70,19 @@ the way Jarl and Steward are.
 - **Warnings about the machine:** while `nftables.service` is enabled
   (it loads `/etc/nftables.conf` at boot, and flushes every table when
   stopped), and while the table an import came from still applies.
+- **Statistics** (Traffic → Statistics, `hc stats`, `/api/v1/stats/...`):
+  traffic per interface and zone, drops by chain and when they happen,
+  hits per rule and the rules that matched nothing, the sources blocked
+  most and what they tried, connections against the kernel's limit, each
+  quota's use, and housecarld's and the host's memory and load, sampled
+  every minute into a database of their own (`stats.db`, kept 7 days per
+  minute, 90 days per hour, 2 years per day). On by default; traffic per
+  inside device is a setting (`--stats-per-device`) that adds counters
+  to the forward chain. Two charts on the dashboard; a warning when the
+  connection tracking table is nearly full (docs/stats.md).
+- Set names starting with `meter_`, `conn_`, `quota_` or `stats_` are
+  refused (`set_name_reserved`): Housecarl's own sets use them, and a set
+  of yours named so could clash with them.
 - **The zone matrix** (Matrix, `hc matrix`): what the rules do to new
   connections between zones and to and from the firewall, worked out
   from the rules in order, with each pair's exceptions; rules about