feat(mounts): P2 — read-write REST for external mounts

Adds full CRUD on mount contents, mirroring the P1 read pattern (handlers/
services classify; authorization stays in the service via the mount-root
folder grant; the provider does the I/O).

- mkdir / rename / delete / move-within branch inside FolderService and
  FileManagementService (router injected into both)
- streaming upload via a new ExternalUploadService: the upload handler detects a
  mount destination BEFORE the CAS ingest and streams the multipart body
  straight to the provider (no BLAKE3/dedup). `write_stream` now takes a
  lifetime-bound boxed stream so the borrowing multipart field can be passed
  without buffering.
- deletes on mounts are permanent (no trash): the trash-first folder handler
  routes `ext:` ids straight to the provider delete; file delete goes through the
  branched delete_and_cleanup
- cross-backend move/copy (mount ↔ native, or between mounts) is forbidden
  (UnsupportedOperation); the mount root itself cannot be renamed/moved/deleted
- every mutation emits a `target:"audit" event="external_mount.write"` line
- shared mount_dto builders synthesize FolderDto/FileDto from a provider MountStat

Tests: 529 unit + integration tests for mkdir/rename/delete, file rename/delete,
streaming upload, cross-boundary forbid, and stranger-denied — all against real
Postgres + a real provider (testcontainers).

src/interfaces/api/handlers/file_handler.rs

@@ -240,6 +240,35 @@ impl FileHandler {
                     }
                 }
 
+                // ── External mount destination? Stream to the provider ──
+                // Detected BEFORE the CAS ingest so the bytes never touch
+                // BLAKE3/dedup. Authorization happens inside the service.
+                if let Some(ref fid) = folder_id {
+                    let (mount_cfg, parent_node) = match state.mount_router.classify(fid) {
+                        ResolvedId::MountRoot { cfg } => (Some(cfg), NodeId::default()),
+                        ResolvedId::MountChild { cfg, node_id } => (Some(cfg), node_id),
+                        ResolvedId::Regular => (None, NodeId::default()),
+                    };
+                    if let Some(cfg) = mount_cfg {
+                        use futures::StreamExt;
+                        let body: crate::application::ports::external_mount_ports::MountByteStream<
+                            '_,
+                        > = Box::pin(
+                            upload_ingest::multipart_field_stream(field)
+                                .map(|r| r.map_err(|e| std::io::Error::other(e.to_string()))),
+                        );
+                        return match state
+                            .applications
+                            .external_upload_service
+                            .write_file(&cfg, &parent_node, &filename, body, auth_user.id)
+                            .await
+                        {
+                            Ok(file) => Ok((file, String::new())),
+                            Err(err) => Err(Self::domain_error_response(err)),
+                        };
+                    }
+                }
+
                 // ── Stream the field into the CDC chunk store ────────
                 // Chunking (FastCDC) + hashing (BLAKE3) + dedup checks +
                 // MIME sniffing all happen while the bytes arrive; chunks