feat: the admin Overview shows who is listening, downloads, the library and its disk, and the server's version, uptime and recent errors; a download waiting on you, a client not answering or a full disk count in its verdict (M504 #5627 #5628 #5629 #5630)

The version moves to internal/buildinfo, so /rest/ping reports the build
instead of "dev".

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

internal/buildinfo/buildinfo.go

@@ -0,0 +1,33 @@
+// Package buildinfo says which build is running and since when. It is the
+// one place the version lives, so /healthz, the Subsonic envelope and the
+// admin Overview cannot disagree about it.
+package buildinfo
+
+import "time"
+
+// Version is the build's own version name — YYYY.MM.DD.HHMM, derived from
+// the commit it was built from by ci/version.sh. Defaults to "dev" for local
+// builds; overridden at link time via:
+//
+//	-ldflags="-X 'git.fabledsword.com/bvandeusen/minstrel/internal/buildinfo.Version=2026.09.10.1449'"
+//
+// release.yml passes it through the MINSTREL_VERSION build-arg → Dockerfile
+// ldflag.
+//
+// This carried the literal strings "main" and "dev" until 2026-09-10, which
+// made every image on a channel report the same thing forever. It stopped
+// being cosmetic when :vYYYY.MM.DD.HHMM image tags were retired (family rule
+// 145): this is now the ONLY thing that says which build is running.
+var Version = "dev"
+
+// Channel is which line this build came off — "stable" or "dev", or "local"
+// for a plain `docker build`.
+//
+// A SIBLING FIELD, never a suffix inside Version (family rule 149). The same
+// commit built on both lanes reports the same version and differs only here;
+// folding the two together is what makes a version string stop being
+// comparable.
+var Channel = "local"
+
+// Started is when this process started, for the Overview's uptime (M504).
+var Started = time.Now()