split into a flake with separable collectors/server modules

Replaces the single lgtm.nix with a flake exposing two NixOS modules under
services.telemetry, so the web services can move into a container declared in
the consuming config rather than one hardcoded here.

  modules/collectors.nix  host-side: node + smartctl exporters, the ZFS and
                          SMART textfile collector timers, Alloy journal
                          shipping. Must run on the host -- reads /proc, /sys,
                          raw block devices and ZFS ioctls.
  modules/server.nix      Prometheus, Loki, Grafana, optional Caddy.
  modules/common.nix      ports, shared by both halves.
  modules/dashboards.nix  jq fixup and grafana.com fetches.
  modules/packages.nix    the two collectors as ordinary packages.

Modules are exported as paths, not functions, so they evaluate with the
consumer's nixpkgs; the flake's own input only affects packages/checks/devShells.

Multi-host support. collectorHosts is a list, both scrape jobs relabel
__address__ into a `host` label, and all 133 queries in the two generated
dashboards are scoped to host=~"$host" behind a single-select Host picker.
`instance` could not serve as that key: one machine is scraped twice on
different ports (textfile metrics on :9100, smartctl_* on :9633) and the
recording rules join across that boundary.

Grafana's secret key becomes a plain secretKeyFile path wired through
LoadCredential, with an assertion -- nixpkgs 26.05 dropped the default. No
sops-nix input.

nix flake check runs promtool over the rules, diffs the committed dashboard
JSON against what the generators produce, and evaluates both modules.

Also fixes smart-selftest emitting a placeholder identity line when smartctl
cannot open a device. smartctl exits 0 with well-formed but empty JSON in that
case, so "SSD - unknown model - no serial" was reaching a dashboard row title;
it now emits nothing and lets the smartctl:ident fallback apply.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JKczN1f66kmLxNBLX37voQ

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