feat(observability): per-GPU metrics for the mixed AMD/NVIDIA fleet

Add a host-process GPU exporter, scrape it on both servers, and graph it
on the host-system dashboard. The fleet has four GPUs across three vendors
of interest (2x R9700 on Dankserver, the 8060S iGPU + RTX 3090 on Dankclaw)
and nothing exposed them until now.

scripts/gpu-exporter.py reads each AMD card's amdgpu sysfs
(gpu_busy_percent / mem_info_vram_* / hwmon temp1_input / power1_average)
plus one nvidia-smi --query-gpu CSV, and emits unified gpu_* Prometheus
families (utilization_percent, memory_used/total_bytes, temperature_celsius,
power_watts) labelled gpu/vendor/model. Absent hardware yields ABSENT series,
never a fabricated 0: the R9700s expose no sysfs power, so they simply carry
no gpu_power_watts line. Both sources are injectable, so the seam runs on a
GPU-less dev box; live-verified on Dankserver's real sysfs.

It runs as a plain user unit (not a quadlet): it must read the host's own DRM
sysfs and shell to nvidia-smi, which a container cannot do without GPU
passthrough. Same posture as signal-relay. The base unit binds loopback; the
deploy drop-in appends the host LAN address, never a wildcard (matching
node-exporter/cadvisor). deploy_gpu_exporter installs/enables/health-gates it
on both hosts and is wired into the planner (both targets), the test-changed
router, and two Prometheus scrape jobs using the node/cadvisor reach rules
(gateway on Dankclaw, LAN on Dankserver).

shared/prometheus/prometheus.yml

@@ -191,3 +191,25 @@ scrape_configs:
         action: keep
       - regex: 'id|name|image|container_label.*|machine_id|boot_id'
         action: labeldrop
+
+  # Per-GPU metrics (gpu_utilization_percent / gpu_memory_used_bytes /
+  # gpu_memory_total_bytes / gpu_temperature_celsius / gpu_power_watts) from the
+  # host-process gpu-exporter (scripts/gpu-exporter.py). One job per host, same
+  # reach rules as node/cadvisor: Dankclaw is scraped at the podman gateway
+  # because the netavark hairpin quirk makes Prometheus fail to dial its own
+  # host LAN IP; Dankserver is plain LAN. The gpu label (PCI BDF on AMD, index on
+  # NVIDIA) + vendor/model come from the exporter, so the GPU row on the
+  # host-system dashboard filters by host. Power is emitted only where readable
+  # (NVIDIA + the Dankclaw iGPU); the R9700s have no sysfs power, so they simply
+  # have no gpu_power_watts series — no fabricated zero.
+  - job_name: "gpu-dankclaw"
+    static_configs:
+      - targets: ["host.containers.internal:9401"]
+        labels:
+          host: dankclaw
+
+  - job_name: "gpu-dankserver"
+    static_configs:
+      - targets: ["192.168.0.235:9401"]
+        labels:
+          host: dankserver