grafana: a forge board, honest about being an inventory board

Adds the fourth per-service board. Unlike its three siblings it answers
"what does the forge CONTAIN", not "is the forge healthy" -- forgejo's
exporter publishes a census of stored objects and no request, error or
latency series at all.

That distinction is the board's own description rather than something a
reader has to infer, because a fourth per-service board that silently
answers a different question than the other three is worse than no board:
someone checks it during an incident and reads a flat line as calm.

Panels: repositories, users, orgs, issues open/closed, releases, the
open-vs-closed pair over time, repos-and-users together (they move in
lockstep when an agent is created, so divergence is the signal),
discussion volume, and the webhook/hooktask pair.

One panel ships deliberately EMPTY: issues-by-label. The metric is
enabled in the forge's config but forgejo reads its [metrics] section at
startup, so the series does not exist until that process restarts -- and
a config merge alone does not restart it. The panel title says so, the
description says how to tell "not deployed" from "no labelled issues",
and `noValue` says it again in the graph itself. An empty panel with no
explanation is the failure this board is trying not to be.

Gated: every panel's metric confirmed to exist in the store, with a
bogus name checked absent in the same run so the check can fail, plus a
known-positive control against the label-values query itself -- a
download that silently returned nothing would otherwise report every
metric missing. Gate kept as verify-3494-forge-board.sh.

nix/host-modules/swarm-grafana.nix

@@ -61,6 +61,7 @@ let
         "agents.json"
         "authelia.json"
         "claude-usage.json"
+        "forge.json"
         "logstore.json"
         "metricstore.json"
         "queue.json"