Add Metrics tab with historical server utilization and map capacity

docs/metrics.md

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+# Metrics
+
+Metrics is a primary navigation tab at `/metrics` (keyboard shortcut: `g m`).
+Choose the past 24 hours, 7 days, or 30 days. The page refreshes every minute.
+
+- Utilized means a successful status response reported at least one player.
+- Idle means a successful response reported zero players.
+- Unknown covers failed checks, provisioning, and unavailable containers; it is excluded from utilization percentages.
+- Player-hours integrate the reported player count over observed time. Bots are included if the server counts them as players.
+- All lobbies full measures map wall time when every tracked lobby in that game/map group has a known capacity and is full. Missing capacity or status makes that interval unknown. This is a capacity-pressure indicator, not a count of rejected joins or unmet demand.
+
+Observations are recorded by the existing status worker. Each remains valid until the next collection, up to the 45-second freshness limit. Longer manager outages are excluded. Charts split time at UTC hour/day boundaries; partial periods remain partial. Map variants use the configured variant when it matches the reported map.
+
+Capacity comes from `sv_maxclients` / `sv_maxClients` in the status response. When unavailable, the UI displays unknown rather than assuming a four-player limit. Capacity coverage is the map time for which every tracked lobby reported both occupancy and capacity.
+
+Collection begins after this feature is installed; previous occupancy cannot be reconstructed. Samples are retained for 31 days and pruned during collection. They survive removal of a server and contain no player identities. The additive database table is created automatically on startup.