feat(mounts): external file mounts P1 — pluggable provider + read-only REST

Adds the foundation for external file mounts: admin-configured backends
(raw host filesystem in v1; sftp/webdav/… as future provider kinds) surfaced
as a folder inside a user's drive. Mount contents are virtual/live-passthrough
— read straight from the backend, never stored in storage.files — and are a
deliberately separate, limited storage type (no dedup/sharing/trash/search).
The feature is dark by default (OXICLOUD_ENABLE_EXTERNAL_MOUNTS=false).

P1 scope (this PR): data model, the pluggable provider abstraction, and the
read-only REST surface (mount listing + download). Read-write (P2),
WebDAV/NextCloud path resolution (P3), and the admin UI (P4) follow.

Core model
- Mount root = a real storage.folders row; authorization for everything inside
  collapses onto that folder UUID (ltree-ancestry grant cascade).
- Children are virtual, addressed by ext:<mount_id>:<base64url(node_id)> where
  node_id is provider-owned and opaque to the rest of the system.
- A lock-free (arc-swap) MountRegistry maps mount-root UUID -> provider; a thin
  MountRouter::classify() is the single cheap hook handlers call before parsing
  an id as a UUID. With no mounts configured it always returns Regular, so
  existing code paths are unchanged.

Added
- migrations/20260805000000_external_mounts.sql (storage.external_mounts, kind + config JSONB)
- domain/services/external_mount_id (id envelope + virtual etags)
- application/ports/external_mount_ports (ExternalMountProvider, MountProviderFactory, repo port)
- infrastructure local_fs_mount_provider (tokio::fs, symlink-escape-safe) + factory
- application MountRegistry + MountRouter, pg ExternalMountRepository
- DI wiring (AppState.mount_router), FeaturesConfig.enable_external_mounts
- listing branch (FolderService::list_mount_dir_with_perms + folder_handler) and
  download branch (FileRetrievalService stat/open mount methods + file_handler)

Authorization stays in the service layer (authz.require(Resource::Folder(mount_id)));
handlers only classify. Cross-backend operations are out of scope for P1.

Tests: 529 unit tests + 5 testcontainers integration tests (real Postgres 17),
including end-to-end authorization (owner allowed, stranger denied). Line
coverage of the new modules is 84–100% (cargo-llvm-cov). Known gap:
file_handler::download_mount_file (HTTP glue) needs a full-app test (P4).

src/infrastructure/services/mount_provider_factory.rs

@@ -0,0 +1,122 @@
+//! The single place external mount provider kinds are registered.
+//!
+//! Adding a new backend (`sftp`, `webdav`, …) is: implement
+//! [`ExternalMountProvider`](crate::application::ports::external_mount_ports::ExternalMountProvider)
+//! and add one arm to [`DefaultMountProviderFactory::build`]. Nothing else in the
+//! router / listing / authz / path-resolution layers changes.
+
+use std::sync::Arc;
+
+use async_trait::async_trait;
+
+use crate::application::ports::external_mount_ports::{
+    ExternalMountProvider, MountProviderFactory,
+};
+use crate::domain::errors::DomainError;
+use crate::infrastructure::services::local_fs_mount_provider::LocalFsMountProvider;
+
+/// Default factory: knows the built-in provider kinds.
+#[derive(Default)]
+pub struct DefaultMountProviderFactory;
+
+impl DefaultMountProviderFactory {
+    /// Construct the factory.
+    pub fn new() -> Self {
+        Self
+    }
+}
+
+#[async_trait]
+impl MountProviderFactory for DefaultMountProviderFactory {
+    async fn build(
+        &self,
+        kind: &str,
+        config: &serde_json::Value,
+    ) -> Result<Arc<dyn ExternalMountProvider>, DomainError> {
+        match kind {
+            "local_fs" => {
+                let path = config.get("path").and_then(|v| v.as_str()).ok_or_else(|| {
+                    DomainError::validation_error(
+                        "local_fs mount config requires a string \"path\"",
+                    )
+                })?;
+                let read_only = config
+                    .get("read_only")
+                    .and_then(|v| v.as_bool())
+                    .unwrap_or(false);
+                let provider = LocalFsMountProvider::new(path, read_only)?;
+                Ok(Arc::new(provider))
+            }
+            other => Err(DomainError::operation_not_supported(
+                "ExternalMount",
+                format!("unknown mount provider kind: {other}"),
+            )),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::domain::errors::{DomainError, ErrorKind};
+
+    /// `Arc<dyn ExternalMountProvider>` isn't `Debug`, so `unwrap_err` won't
+    /// compile — extract the error by matching instead.
+    fn expect_err(r: Result<Arc<dyn ExternalMountProvider>, DomainError>) -> DomainError {
+        match r {
+            Ok(_) => panic!("expected an error"),
+            Err(e) => e,
+        }
+    }
+
+    #[tokio::test]
+    async fn builds_local_fs_provider_from_valid_config() {
+        let dir = tempfile::tempdir().unwrap();
+        let factory = DefaultMountProviderFactory::new();
+        let cfg = serde_json::json!({ "path": dir.path().to_str().unwrap() });
+        let provider = factory.build("local_fs", &cfg).await.expect("builds");
+        assert_eq!(provider.kind(), "local_fs");
+    }
+
+    #[tokio::test]
+    async fn local_fs_honours_read_only_flag() {
+        let dir = tempfile::tempdir().unwrap();
+        let factory = DefaultMountProviderFactory::new();
+        let cfg = serde_json::json!({ "path": dir.path().to_str().unwrap(), "read_only": true });
+        let provider = factory.build("local_fs", &cfg).await.unwrap();
+        assert!(provider.capabilities().read_only);
+    }
+
+    #[tokio::test]
+    async fn unknown_kind_is_unsupported() {
+        let factory = DefaultMountProviderFactory::new();
+        let err = expect_err(factory.build("sftp", &serde_json::json!({})).await);
+        assert_eq!(err.kind, ErrorKind::UnsupportedOperation);
+    }
+
+    #[tokio::test]
+    async fn local_fs_missing_path_is_validation_error() {
+        let factory = DefaultMountProviderFactory::new();
+        let err = expect_err(
+            factory
+                .build("local_fs", &serde_json::json!({ "read_only": true }))
+                .await,
+        );
+        assert_eq!(err.kind, ErrorKind::InvalidInput);
+    }
+
+    #[tokio::test]
+    async fn local_fs_nonexistent_path_errors() {
+        let factory = DefaultMountProviderFactory::new();
+        let err = expect_err(
+            factory
+                .build(
+                    "local_fs",
+                    &serde_json::json!({ "path": "/no/such/dir/xyz123" }),
+                )
+                .await,
+        );
+        // Propagated from LocalFsMountProvider::new (canonicalize failure).
+        assert_eq!(err.kind, ErrorKind::InternalError);
+    }
+}