feat: Phase 3 - architecture & maintainability

- Split 1474-line dashboard.go into 5 domain files (clients, providers, users, system)
- Unit tests for ModelRegistry.FindModel and CalculateCost
- go mod tidy + verify (deps clean)
- .gitignore excludes tool cache dirs (.pi-lens/, .opencode/)

internal/server/system.go

@@ -0,0 +1,156 @@
+package server
+
+import (
+	"database/sql"
+	"fmt"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/google/uuid"
+	"golang.org/x/crypto/bcrypt"
+	"gophergate/internal/db"
+	"gophergate/internal/models"
+	"gophergate/internal/utils"
+	"log/slog"
+
+	"github.com/shirou/gopsutil/v3/cpu"
+
+func (s *Server) handleSystemHealth(c *gin.Context) {
+	c.JSON(http.StatusOK, SuccessResponse(gin.H{
+		"status": "ok",
+		"components": gin.H{
+			"database": "online",
+			"proxy":    "online",
+		},
+	}))
+}
+
+func (s *Server) handleSystemMetrics(c *gin.Context) {
+	v, _ := mem.VirtualMemory()
+	c_usage, _ := cpu.Percent(time.Second, false)
+	d, _ := disk.Usage("/")
+	l, _ := load.Avg()
+	p, _ := process.NewProcess(int32(os.Getpid()))
+	rss, _ := p.MemoryInfo()
+
+	cpuPercent := 0.0
+	if len(c_usage) > 0 {
+		cpuPercent = c_usage[0]
+	}
+
+	c.JSON(http.StatusOK, SuccessResponse(gin.H{
+		"cpu": gin.H{
+			"usage_percent": fmt.Sprintf("%.1f", cpuPercent),
+			"load_average":  []float64{l.Load1, l.Load5, l.Load15},
+		},
+		"memory": gin.H{
+			"used_mb":        v.Used / 1024 / 1024,
+			"total_mb":       v.Total / 1024 / 1024,
+			"usage_percent":  fmt.Sprintf("%.1f", v.UsedPercent),
+			"process_rss_mb": rss.RSS / 1024 / 1024,
+		},
+		"disk": gin.H{
+			"used_gb":       float64(d.Used) / 1024 / 1024 / 1024,
+			"total_gb":      float64(d.Total) / 1024 / 1024 / 1024,
+			"usage_percent": fmt.Sprintf("%.1f", d.UsedPercent),
+		},
+		"connections": gin.H{
+			"db_active":           s.database.Stats().OpenConnections,
+			"websocket_listeners": s.hub.GetClientCount(),
+		},
+	}))
+}
+
+func (s *Server) handleGetSettings(c *gin.Context) {
+	providerCount := 0
+	modelCount := 0
+	s.registryMu.RLock()
+	if s.registry != nil {
+		providerCount = len(s.registry.Providers)
+		for _, p := range s.registry.Providers {
+			modelCount += len(p.Models)
+		}
+	}
+
+	c.JSON(http.StatusOK, SuccessResponse(gin.H{
+		"server": gin.H{
+			"version":     "1.0.0-go",
+			"auth_tokens": s.cfg.Server.AuthTokens,
+		},
+		"database": gin.H{
+			"type": "sqlite",
+			"path": s.cfg.Database.Path,
+		},
+		"registry": gin.H{
+			"provider_count": providerCount,
+			"model_count":    modelCount,
+		},
+	}))
+}
+
+func (s *Server) handleCreateBackup(c *gin.Context) {
+	// Simplified backup response
+	c.JSON(http.StatusOK, SuccessResponse(gin.H{
+		"backup_id": fmt.Sprintf("backup-%d.db", time.Now().Unix()),
+		"status":    "created",
+	}))
+}
+
+func (s *Server) handleGetLogs(c *gin.Context) {
+	var logs []db.LLMRequest
+	err := s.database.Select(&logs, "SELECT * FROM llm_requests ORDER BY timestamp DESC LIMIT 100")
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, ErrorResponse(err.Error()))
+		return
+	}
+
+	// Format for UI
+	type UILog struct {
+		Timestamp string `json:"timestamp"`
+		ClientID  string `json:"client_id"`
+		Provider  string `json:"provider"`
+		Model     string `json:"model"`
+		Tokens    int    `json:"tokens"`
+		Status    string `json:"status"`
+		Duration  int    `json:"duration"`
+	}
+
+	uiLogs := make([]UILog, len(logs))
+	for i, l := range logs {
+		clientID := "unknown"
+		if l.ClientID != nil {
+			clientID = *l.ClientID
+		}
+		provider := "unknown"
+		if l.Provider != nil {
+			provider = *l.Provider
+		}
+		model := "unknown"
+		if l.Model != nil {
+			model = *l.Model
+		}
+		tokens := 0
+		if l.TotalTokens != nil {
+			tokens = *l.TotalTokens
+		}
+		duration := 0
+		if l.DurationMS != nil {
+			duration = *l.DurationMS
+		}
+
+		uiLogs[i] = UILog{
+			Timestamp: l.Timestamp.Format(time.RFC3339),
+			ClientID:  clientID,
+			Provider:  provider,
+			Model:     model,
+			Tokens:    tokens,
+			Status:    l.Status,
+			Duration:  duration,
+		}
+	}
+
+	c.JSON(http.StatusOK, SuccessResponse(uiLogs))
+}