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/api.md

@@ -99,7 +99,8 @@ Creating answers 201, other changes 200.
  "builtins": {"loopback": true, "established": true, "drop_invalid": true, "icmpv6": true,
               "ping_internal": true, "dhcpv6_client": true, "antispoof": true, "mss_clamp": false},
  "own_prefixes": ["2001:db8::/48"], "antispoof_v4": ["10.0.0.0/8", "..."], "antispoof_v6": ["fc00::/7", "..."],
- "flowtable_devices": ["wan", "eth1"], "probation_seconds": 120, "manual_confirm": false}
+ "flowtable_devices": ["wan", "eth1"], "probation_seconds": 120, "manual_confirm": false,
+ "stats_per_device": false}
 ```
 
 Lists replace the old ones. `probation_seconds` 0 is the default (120).
@@ -145,6 +146,12 @@ A source NAT rule is a match (as a rule's, without the rule's other
 fields) with `"to"` (empty: masquerade), a redirect a match with
 `"to_port"`. Times are seconds (`timeout`).
 
+### Statistics
+
+`GET /stats/status`, `/stats/summary`, `/stats/timeline`, `/stats/top`,
+`/stats/hits` and `/stats/heatmap` (`log:read`): see
+[stats.md](stats.md#the-api).
+
 ### Changesets
 
 Changes staged to be reviewed and applied together ([design.md](design.md#reviewing-changes-changesets)).