feat(tickets): ajoute les pièces jointes sur tickets (#108)

Stockage flat côté ticket + métadonnées d'attachments, lecture exposée côté
backend/MCP, et UI minimale de liste/ajout dans TicketDetail. Traverse le
domaine (Issue, ports), l'application (usecases + assistant de ticket), les
adaptateurs infra/MCP (issues store, orchestrateur), les DTO backend/web-
server/app-tauri, et le frontend (domain/ports/adapters/hooks/UI).

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

frontend/src/adapters/http/streamGateways.ts

@@ -29,6 +29,7 @@ import type {
   Sprint,
   TerminalSession,
   Ticket,
+  TicketAttachmentContent,
   TicketBulkResult,
   TicketCarnet,
   TicketChat,
@@ -122,6 +123,11 @@ export class HttpSystemGateway implements SystemGateway {
     return unsupportedOnWeb("Native file picker");
   }
 
+  pickFile(): Promise<string | null> {
+    // Desktop-only, same rationale as `pickFolder`.
+    return unsupportedOnWeb("Native file picker");
+  }
+
   onAppExitWorkGuard(
     _handler: (state: AppExitWorkGuardState) => void,
   ): Promise<Unsubscribe> {
@@ -321,6 +327,36 @@ export class HttpTicketGateway implements TicketGateway {
       request: { projectId, ref, carnet, expectedVersion },
     });
   }
+  addAttachment(
+    projectId: string,
+    ref: string,
+    path: string,
+    expectedVersion: number,
+    mime?: string | null,
+  ): Promise<Ticket> {
+    return this.http.invoke<Ticket>("ticket_attachment_add", {
+      request: { projectId, ref, path, expectedVersion, mime },
+    });
+  }
+  readAttachment(
+    projectId: string,
+    ref: string,
+    attachmentId: string,
+  ): Promise<TicketAttachmentContent> {
+    return this.http.invoke<TicketAttachmentContent>("ticket_attachment_read", {
+      request: { projectId, ref, attachmentId },
+    });
+  }
+  markAttachmentSummarized(
+    projectId: string,
+    ref: string,
+    attachmentId: string,
+    expectedVersion: number,
+  ): Promise<Ticket> {
+    return this.http.invoke<Ticket>("ticket_attachment_mark_summarized", {
+      request: { projectId, ref, attachmentId, expectedVersion },
+    });
+  }
   link(
     projectId: string,
     ref: string,