ops: Signal disk-full and eviction capacity pressure

When the disk (or memory) tier is at cap or the volume returns ENOSPC,
ops currently look like random misses with no clear "we are dropping
data." Count those events as capacity_pressure_events on /metrics and
log tier plus reason so operators can tell capacity pressure from a
cold cache, without changing the existing evictions counter.

Link: #36

steamcache/steamcache_test.go

@@ -1104,6 +1104,9 @@ func TestDiskOnlyDelayedAttach(t *testing.T) {
 	if !bytes.Contains(rec.Body.Bytes(), []byte("disk_tier_ready 1")) {
 		t.Errorf("WriteText output missing \"disk_tier_ready 1\": %q", rec.Body.String())
 	}
+	if !bytes.Contains(rec.Body.Bytes(), []byte("capacity_pressure_events")) {
+		t.Errorf("WriteText output missing capacity_pressure_events: %q", rec.Body.String())
+	}
 }
 
 // TestDiskTierSignalMemoryOnly covers memory-only mode: DiskTierReady=1 (N/A, not