feat(admin): add LLM call logs, system info, plugin list pages

- ai-core/llm: ring-buffer call logger with stats
- Gateway: /api/v1/admin/llm-calls, /system, /plugins, /sessions endpoints
- Frontend: LLMCallsPage (table + stats), SystemPage (version/plugins/sessions)
- Admin nav expanded to 7 items

backend/services/ai-core/llm/logger.go

@@ -0,0 +1,84 @@
+package llm
+
+import (
+	"sync"
+	"time"
+)
+
+type CallRecord struct {
+	ID           string    `json:"id"`
+	Time         time.Time `json:"time"`
+	Model        string    `json:"model"`
+	Purpose      string    `json:"purpose"`
+	PromptTokens int       `json:"prompt_tokens"`
+	CompTokens   int       `json:"completion_tokens"`
+	DurationMs   int64     `json:"duration_ms"`
+	Success      bool      `json:"success"`
+	Error        string    `json:"error,omitempty"`
+}
+
+type Logger struct {
+	mu      sync.RWMutex
+	records []CallRecord
+	max     int
+	index   int
+	total   int64
+}
+
+func NewLogger(max int) *Logger { return &Logger{records: make([]CallRecord, max), max: max} }
+
+func (l *Logger) Record(r CallRecord) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	r.ID = time.Now().Format("150405") + "-" + r.Purpose
+	l.records[l.index%l.max] = r
+	l.index++
+	l.total++
+}
+
+func (l *Logger) Recent(n int) []CallRecord {
+	l.mu.RLock()
+	defer l.mu.RUnlock()
+	if n > l.max { n = l.max }
+	count := l.index
+	if count > l.max { count = l.max }
+	if n > count { n = count }
+	result := make([]CallRecord, n)
+	for i := 0; i < n; i++ {
+		idx := (l.index - 1 - i + l.max) % l.max
+		if idx < l.max { result[n-1-i] = l.records[idx] }
+	}
+	return result
+}
+
+func (l *Logger) RecentMaps(n int) []map[string]any {
+	records := l.Recent(n)
+	result := make([]map[string]any, len(records))
+	for i, r := range records {
+		result[i] = map[string]any{
+			"id": r.ID, "time": r.Time.Format(time.RFC3339), "model": r.Model,
+			"purpose": r.Purpose, "prompt_tokens": r.PromptTokens, "completion_tokens": r.CompTokens,
+			"duration_ms": r.DurationMs, "success": r.Success, "error": r.Error,
+		}
+	}
+	return result
+}
+
+func (l *Logger) Stats() map[string]any {
+	l.mu.RLock()
+	defer l.mu.RUnlock()
+	var calls, succ int64
+	var dur, tok int64
+	count := l.index
+	if count > l.max { count = l.max }
+	for i := 0; i < count; i++ {
+		r := l.records[(l.index-1-i+l.max)%l.max]
+		calls++
+		if r.Success { succ++ }
+		dur += r.DurationMs
+		tok += int64(r.PromptTokens + r.CompTokens)
+	}
+	avg := 0
+	if calls > 0 { avg = int(dur / calls) }
+	return map[string]any{"total_calls": calls, "success_calls": succ, "avg_duration_ms": avg, "total_tokens": tok}
+}