feat(server/diagnostics): device debug-reporting ingest + admin timeline + retention (M9)

New diagnostic_events table + per-account users.debug_mode_enabled flag.
When an account's flag is on, its client(s) POST a batch timeseries of
connectivity / UPnP-sync / power / lifecycle events to /api/diagnostics
(no-op 204 when off, kind whitelist mirrors the CHECK constraint).

Admin surface: GET /api/admin/diagnostics (optional account/device/kind/
time-window filters, RFC3339-or-epoch-ms, export-sized paging) + a
/diagnostics/devices overview + PUT /api/admin/users/{id}/debug-mode to
flip an account remotely while a bug is live. debug_mode_enabled is now
exposed on /api/me (client gate) and the admin user views.

Retention: a 30-day gc-worker sweep (GcPruneDiagnostics), keyed on the
server clock so a skewed device clock can't keep rows alive.

Refs Scribe M9 (#119), tasks #1172 #1173.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K55iTxn95BtshocgdE1shW

internal/api/admin_users.go

@@ -22,6 +22,7 @@ type adminUserView struct {
 	DisplayName         *string `json:"display_name"`
 	IsAdmin             bool    `json:"is_admin"`
 	AutoApproveRequests bool    `json:"auto_approve_requests"`
+	DebugModeEnabled    bool    `json:"debug_mode_enabled"`
 	CreatedAt           string  `json:"created_at"`
 }
 
@@ -44,6 +45,7 @@ func (h *handlers) handleAdminListUsers(w http.ResponseWriter, r *http.Request)
 			DisplayName:         row.DisplayName,
 			IsAdmin:             row.IsAdmin,
 			AutoApproveRequests: row.AutoApproveRequests,
+			DebugModeEnabled:    row.DebugModeEnabled,
 			CreatedAt:           row.CreatedAt.Time.UTC().Format(time.RFC3339),
 		}
 		out = append(out, v)
@@ -120,6 +122,7 @@ func (h *handlers) handleUpdateUserAdmin(w http.ResponseWriter, r *http.Request)
 		DisplayName:         updated.DisplayName,
 		IsAdmin:             updated.IsAdmin,
 		AutoApproveRequests: updated.AutoApproveRequests,
+		DebugModeEnabled:    updated.DebugModeEnabled,
 		CreatedAt:           updated.CreatedAt.Time.UTC().Format(time.RFC3339),
 	})
 }
@@ -189,6 +192,7 @@ func (h *handlers) handleAdminCreateUser(w http.ResponseWriter, r *http.Request)
 		DisplayName:         user.DisplayName,
 		IsAdmin:             user.IsAdmin,
 		AutoApproveRequests: user.AutoApproveRequests,
+		DebugModeEnabled:    user.DebugModeEnabled,
 		CreatedAt:           user.CreatedAt.Time.UTC().Format(time.RFC3339),
 	})
 }
@@ -329,6 +333,7 @@ func (h *handlers) handleAdminAutoApproveToggle(w http.ResponseWriter, r *http.R
 		DisplayName:         updated.DisplayName,
 		IsAdmin:             updated.IsAdmin,
 		AutoApproveRequests: updated.AutoApproveRequests,
+		DebugModeEnabled:    updated.DebugModeEnabled,
 		CreatedAt:           updated.CreatedAt.Time.UTC().Format(time.RFC3339),
 	})
 }