feat(api): profil machine éditable, sonde, et réglages globaux apt-cacher-ng

- machines : updateMachine (PATCH /machines/:id) + POST /machines/:id/probe
  (sonde synchrone → faits + proposition de correction) ; MachineView expose
  machineKind/virtualization ; CreateMachineInput accepte aptProxyMode persistent
- app_settings (clé/valeur, migration 0006) + service appSettings :
  défaut apt-cacher-ng (mode + url) ; applyProxyToAllMachines
- routes /settings : GET, PUT /apt-proxy, POST /apt-proxy/apply-all

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/db/schema.ts

@@ -312,3 +312,10 @@ export const actionRequests = sqliteTable("action_requests", {
   executionId: text("execution_id"),
   expiresAt: text("expires_at"),
 });
+
+// Réglages globaux de l'application (clé/valeur). Ex. proxy APT par défaut.
+export const appSettings = sqliteTable("app_settings", {
+  key: text("key").primaryKey(),
+  value: text("value"),
+  updatedAt: text("updated_at").notNull(),
+});