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>

internal/validate/validate_test.go

@@ -65,6 +65,15 @@ func TestCodes(t *testing.T) {
 		}},
 		{"set_entry", func(st *model.State) { st.Sets[0].Entries = []string{"10.0.10.5/24"} }},
 		{"set_empty", func(st *model.State) { st.Sets[0].Entries = nil }},
+		{"set_name_reserved", func(st *model.State) { st.Sets[0].Name = "stats_up" }},
+		{"stats_flowtable", func(st *model.State) {
+			st.Settings.StatsPerDevice = true
+			st.Settings.FlowtableDevices = []string{"wan", "eth1"}
+		}},
+		{"stats_no_external", func(st *model.State) {
+			st.Settings.StatsPerDevice = true
+			st.Zones[0].External = false
+		}},
 		{"set_timeout", func(st *model.State) { st.Sets[0].Timeout = 60 }},
 		{"set_dynamic_entries", func(st *model.State) { st.Sets[0].Dynamic = true }},
 		{"service_empty", func(st *model.State) { st.Services[0].Entries = nil }},