fix(audit-2026-06-11/F-FS-002): belt-and-suspenders collection validation on files read/delete

The admin files endpoints hit FsFilesRepo directly (not via
FilesServiceImpl, which validates), so only create/update guarded the
collection — head/get/list/delete built FS paths from an unvalidated
value. Today nothing can store a traversal-shaped collection, but one
future bad migration / restore tool inserting collection='../../etc'
would give the read/delete paths arbitrary host-file reach.

- FsFilesRepo::{head,get,list,delete} now call guard_collection (create
  and update already did).
- The three admin endpoints (list/get/delete) call
  validate_files_collection up front for a clean 422 instead of the
  repo guard surfacing as an opaque 500.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/files_api.rs

@@ -22,7 +22,7 @@ use axum::http::StatusCode;
 use axum::response::{IntoResponse, Json, Response};
 use axum::routing::get;
 use axum::{Extension, Router};
-use picloud_shared::{AppId, Principal};
+use picloud_shared::{validate_files_collection, AppId, Principal};
 use serde::{Deserialize, Serialize};
 use serde_json::json;
 use uuid::Uuid;
@@ -88,11 +88,13 @@ async fn list_files(
         Capability::AppFilesRead(app_id),
     )
     .await?;
-    if q.collection.trim().is_empty() {
-        return Err(FilesApiError::Invalid(
-            "collection must not be empty".into(),
-        ));
-    }
+    // Audit 2026-06-11 (F-FS-002) — the admin endpoints hit the repo
+    // directly (not via FilesServiceImpl, which validates), so a
+    // traversal-shaped collection would otherwise only be caught by the
+    // repo's `guard_collection` as an opaque 500. Validate here for a
+    // clean 422 and as the outer layer of the belt-and-suspenders guard.
+    validate_files_collection(&q.collection)
+        .map_err(|e| FilesApiError::Invalid(e.to_string()))?;
     let page = s
         .files
         .list(
@@ -149,6 +151,7 @@ async fn get_file(
         Capability::AppFilesRead(app_id),
     )
     .await?;
+    validate_files_collection(&collection).map_err(|e| FilesApiError::Invalid(e.to_string()))?;
     let id = Uuid::parse_str(&file_id).map_err(|_| FilesApiError::NotFound)?;
     let meta = s
         .files
@@ -193,6 +196,7 @@ async fn delete_file(
         Capability::AppFilesWrite(app_id),
     )
     .await?;
+    validate_files_collection(&collection).map_err(|e| FilesApiError::Invalid(e.to_string()))?;
     let id = Uuid::parse_str(&file_id).map_err(|_| FilesApiError::NotFound)?;
     if s.files.delete(app_id, &collection, id).await?.is_none() {
         return Err(FilesApiError::NotFound);