Add changesets, bandwidth limits and quotas; generic examples

Changesets, as in Jarl: configuration changes are operations that can be
staged in a changeset, checked as they are added (validation and nft -c),
reviewed with the diff of the ruleset, and applied together as one
history entry, then on probation like any change made over the network.
The web UI collects edits for review by default (pending rows, a bar with
Review and Apply now, a Changes page, a switch for live edits); hc stages
with -C ID or HOUSECARL_CHANGESET (checked like JARL_CHANGESET) and has
hc changeset create/list/show/diff/apply/discard/remove. Undo, import and
takeover refuse a changeset. --dry-run shows the ruleset's diff.

Bandwidth and quotas: rate limits in bytes, per source or destination;
quotas per rule or per address that reset daily, weekly, monthly on a
day, or by hand, with their usage carried across reloads, saved for
reboots, shown by hc quotas and on the rules page, and reset with hc rule
reset-quota. Rules that count traffic are checked before established
connections are accepted (so they see every packet) and can't accept.
Per-address limits group IPv6 by /64 by default (--group-v4/-v6).
Per-source rate meters use update. The importer turns meters, byte rates
and quotas into these fields.

The examples in the web UI, hc's help and the docs use documentation
addresses and generic names instead of the maintainer's network.

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

docs/validation.md

@@ -83,6 +83,15 @@ These apply to rules, source NAT and redirects.
 | `rule_day`, `rule_hours` | error | an unknown day, or hours that aren't `HH:MM`, or only one of them |
 | `rule_limit` | error | a rate that isn't a number per second, minute, hour or day |
 | `rule_conn_limit` | error | a connection limit under 1 |
+| `rule_limit_unit` | error | a rate's unit that isn't packets, bytes, kbytes or mbytes |
+| `rule_limit_bytes` | error | a byte rate nftables can't hold: over 4 GB a second, 293 MB a minute, 4.9 MB an hour or 208 KB a day (the message gives it per second) |
+| `rule_limit_key` | error | a limit both per source and per destination |
+| `rule_quota` | error | a quota under 1 byte |
+| `rule_quota_key` | error | a quota both per source and per destination |
+| `rule_quota_reset` | error | a reset that isn't daily, weekly or monthly; a day with another reset than monthly; a day outside 1-31 |
+| `rule_group` | error | a grouping prefix outside 1-32 (IPv4) or 1-128 (IPv6) |
+| `rule_traffic_accept` | error | a rule with a quota or a byte rate that accepts: it is checked before established connections are accepted, so it can only drop, reject or log |
+| `flowtable_traffic` | warning | a forward rule with a quota or a byte rate while the flowtable is on: it only counts each connection's first packets |
 | `reject_with` | error | `tcp-reset` on a rule that matches more than TCP, or with an action other than reject |
 | `log_prefix` | error | a log prefix over 100 characters |