feat(attachments): fondation pipeline durable d'attachments agent/chat + sandbox-safe (#154, QA verte)

Socle #154 : entité attachment, store, contrat DTO et routage session. Modules : domain/chat_attachment, application/chat_attachments, infrastructure/chat_attachments, app-tauri (commands/lib), backend dto.

crates/backend/src/dto.rs

@@ -2966,6 +2966,79 @@ impl From<TerminalSession> for TerminalSessionDto {
 // Structured chat sessions (§17 — D4)
 // ---------------------------------------------------------------------------
 
+/// Structured attachment intent sent by the chat frontend.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ChatAttachmentInputDto {
+    /// Local source path selected or staged by the driving adapter.
+    pub path: String,
+    /// Optional MIME type known by the frontend/OS picker.
+    #[serde(default)]
+    pub mime: Option<String>,
+    /// Origin kind (`localFile`, `clipboard`, `dragDrop`, `other`).
+    #[serde(default)]
+    pub source_kind: Option<domain::ChatAttachmentSourceKind>,
+}
+
+/// Request for importing durable chat attachments before or during a structured turn.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ImportChatAttachmentsRequestDto {
+    /// Live structured/chat session id.
+    pub session_id: String,
+    /// Legacy local source paths, kept for transition compatibility.
+    #[serde(default)]
+    pub attachment_paths: Vec<String>,
+    /// Structured attachment intents.
+    #[serde(default)]
+    pub attachments: Vec<ChatAttachmentInputDto>,
+}
+
+/// Response returned after durable chat attachment import.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ImportChatAttachmentsResponseDto {
+    /// Imported durable attachments.
+    pub attachments: Vec<ChatAttachmentDto>,
+}
+
+/// Durable chat attachment metadata returned by the backend/application layer.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ChatAttachmentDto {
+    /// Stable attachment id.
+    pub id: String,
+    /// Original display filename.
+    pub filename: String,
+    /// MIME type.
+    pub mime: String,
+    /// Size in bytes.
+    pub size_bytes: u64,
+    /// Origin kind.
+    pub source_kind: domain::ChatAttachmentSourceKind,
+    /// Stored relative path under `.ideai/attachments`.
+    pub storage_path: String,
+    /// Absolute managed path readable by the target agent sandbox.
+    pub readable_path: String,
+    /// Creation time, epoch milliseconds.
+    pub created_at: u64,
+}
+
+impl From<domain::ChatAttachment> for ChatAttachmentDto {
+    fn from(value: domain::ChatAttachment) -> Self {
+        Self {
+            id: value.id.as_str().to_owned(),
+            filename: value.filename,
+            mime: value.mime,
+            size_bytes: value.size_bytes,
+            source_kind: value.source_kind,
+            storage_path: value.storage_path,
+            readable_path: value.readable_path,
+            created_at: value.created_at,
+        }
+    }
+}
+
 /// One incremental chunk of a structured agent reply, streamed over the chat
 /// session's adapter-owned channel. The serialised wire twin of a
 /// [`domain::ports::ReplyEvent`]: the `agent_send` pump maps each turn event to