Add capacity overview with host resource history

docs/native-hosts.md

@@ -233,6 +233,29 @@ systemctl status electric-cherry-update.timer
 journalctl -u electric-cherry-update.service -n 100
 ```
 
+## Capacity overview
+
+Open **Capacity** (`/capacity`) to see the same cached estimates used by collection
+previews. Available totals include hosts ready for the default runtime and latest
+files for the selected game. **All games** counts shared resources once, including
+hosts ready for either game; it is not a guarantee that every game fits every host.
+The host breakdown shows readiness, missing measurements, available ports, learned
+running/startup allowances, and the resource limiting additional servers.
+
+Startup slots and queued jobs are fleet-wide, even with a host or game selected.
+Server estimates use running allowances; provisioning still checks the larger
+startup allowance before starting each job. Port availability uses each host's
+configured bind address. Queued unplaced jobs do not reserve physical resources.
+
+The history graph offers the last hour, 24 hours, or seven days of measured CPU
+and memory headroom, after the usual OS reserve. It averages each host's reports
+within each period, then sums hosts for fleet views. Missing reports appear as gaps.
+Historical measurements cover the selected currently registered hosts and do not
+reconstruct past readiness, port reservations, or startup slots. Reports refresh
+automatically while the page is visible, and historical queries run in a separate
+reader worker. `GET /api/capacity` supports `range=1h|24h|7d`, `host=<host ID>`, and
+`game=all|t5|t6`; host/game selections also remain in the page URL.
+
 ## Identity, transport and credentials
 
 Enrollment tokens are random, hashed at rest, single-use and expire after 15