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/me_profile.go

@@ -35,6 +35,10 @@ type meProfileResp struct {
 	DisplayName *string `json:"display_name"`
 	Email       *string `json:"email"`
 	IsAdmin     bool    `json:"is_admin"`
+	// DebugModeEnabled is the account's diagnostics opt-in. The client
+	// reads it here to decide whether to run the on-device diagnostics
+	// reporter (M9). Admin-set; the client also has a local OFF switch.
+	DebugModeEnabled bool `json:"debug_mode_enabled"`
 }
 
 func (h *handlers) handleUpdateMyProfile(w http.ResponseWriter, r *http.Request) {
@@ -120,10 +124,11 @@ func (h *handlers) handleUpdateMyProfile(w http.ResponseWriter, r *http.Request)
 
 func profileViewFromUser(u dbq.User) meProfileResp {
 	return meProfileResp{
-		ID:          uuidToString(u.ID),
-		Username:    u.Username,
-		DisplayName: u.DisplayName,
-		Email:       u.Email,
-		IsAdmin:     u.IsAdmin,
+		ID:               uuidToString(u.ID),
+		Username:         u.Username,
+		DisplayName:      u.DisplayName,
+		Email:            u.Email,
+		IsAdmin:          u.IsAdmin,
+		DebugModeEnabled: u.DebugModeEnabled,
 	}
 }