feat: show error reason when monitors go DOWN

Propagate check failure reasons through the entire stack:
- Checker captures specific errors (DNS, timeout, HTTP status, SSL, etc.)
- Engine tracks LastError, StatusChangedAt, LastSuccessAt per monitor
- State transitions persisted to new state_changes table
- Detail panel shows error reason, HTTP code, state duration, last
  success time, and last 5 state change events
- Monitor table shows inline error preview for DOWN services
- Alert messages include error reason
- Probe nodes forward error reasons to leader

15 files changed across models, checker, engine, store, TUI, and probes.

internal/server/server.go

@@ -403,9 +403,10 @@ func Start(cfg ServerConfig, s store.Store, eng *monitor.Engine) *http.Server {
 		var req struct {
 			NodeID  string `json:"node_id"`
 			Results []struct {
-				SiteID    int   `json:"site_id"`
-				LatencyNs int64 `json:"latency_ns"`
-				IsUp      bool  `json:"is_up"`
+				SiteID      int    `json:"site_id"`
+				LatencyNs   int64  `json:"latency_ns"`
+				IsUp        bool   `json:"is_up"`
+				ErrorReason string `json:"error_reason"`
 			} `json:"results"`
 		}
 		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
@@ -420,7 +421,7 @@ func Start(cfg ServerConfig, s store.Store, eng *monitor.Engine) *http.Server {
 			if err := s.SaveCheckFromNode(result.SiteID, req.NodeID, result.LatencyNs, result.IsUp); err != nil {
 				log.Printf("Failed to save probe result: %v", err)
 			}
-			eng.IngestProbeResult(req.NodeID, result.SiteID, result.LatencyNs, result.IsUp)
+			eng.IngestProbeResult(req.NodeID, result.SiteID, result.LatencyNs, result.IsUp, result.ErrorReason)
 		}
 		if err := s.UpdateNodeLastSeen(req.NodeID); err != nil {
 			log.Printf("Failed to update node last seen: %v", err)

internal/server/server_test.go

@@ -4,13 +4,14 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
-	"gitea.lerkolabs.com/lerko/uptop/internal/models"
-	"gitea.lerkolabs.com/lerko/uptop/internal/monitor"
 	"net"
 	"net/http"
 	"sync"
 	"testing"
 	"time"
+
+	"gitea.lerkolabs.com/lerko/uptop/internal/models"
+	"gitea.lerkolabs.com/lerko/uptop/internal/monitor"
 )
 
 // --- Mock Store ---
@@ -69,13 +70,15 @@ func (m *mockStore) LoadLogs(int) ([]string, error)           { return nil, nil
 func (m *mockStore) GetAllMaintenanceWindows(int) ([]models.MaintenanceWindow, error) {
 	return nil, nil
 }
-func (m *mockStore) AddMaintenanceWindow(models.MaintenanceWindow) error { return nil }
-func (m *mockStore) EndMaintenanceWindow(int) error                      { return nil }
-func (m *mockStore) DeleteMaintenanceWindow(int) error                   { return nil }
-func (m *mockStore) IsMonitorInMaintenance(int) (bool, error)            { return false, nil }
-func (m *mockStore) GetPreference(string) (string, error)                { return "", nil }
-func (m *mockStore) SetPreference(string, string) error                  { return nil }
-func (m *mockStore) Close() error                                        { return nil }
+func (m *mockStore) AddMaintenanceWindow(models.MaintenanceWindow) error    { return nil }
+func (m *mockStore) EndMaintenanceWindow(int) error                         { return nil }
+func (m *mockStore) DeleteMaintenanceWindow(int) error                      { return nil }
+func (m *mockStore) IsMonitorInMaintenance(int) (bool, error)               { return false, nil }
+func (m *mockStore) GetPreference(string) (string, error)                   { return "", nil }
+func (m *mockStore) SetPreference(string, string) error                     { return nil }
+func (m *mockStore) SaveStateChange(int, string, string, string) error      { return nil }
+func (m *mockStore) GetStateChanges(int, int) ([]models.StateChange, error) { return nil, nil }
+func (m *mockStore) Close() error                                           { return nil }
 
 func (m *mockStore) ExportData() (models.Backup, error) {
 	return models.Backup{