feat: heap convert can create a subfolder under the target

The previous /heaps/{id}/convert dropped photos directly into a chosen
source root, which is rarely what you want — Lightroom-style behaviour
is "make a folder named after the collection inside the library".
Now the dialog lets you do that.

Backend
- HeapConvertBody gains an optional subfolder_name field. Path
  separators and dot-segments are rejected. When set, the handler
  joins it onto the resolved parent_dir, mkdir's it if missing, and
  uses the resulting path as the move/copy destination. Otherwise
  the parent_dir itself is used (unchanged behaviour).
- The Folder DB row for the destination is created via the existing
  scanner get_or_create_folder helper so dedupe + path normalization
  stay consistent across the codebase.
- The target source root id is propagated through both the source-
  root and folder branches so the new Folder row is correctly
  parented when subfolder_name is set on a folder target too.

Frontend
- HeapConvertDialog grows a "Subfolder name" input that prefills
  with the heap name when the dialog opens. Trimmed empty value
  drops directly into the parent. A live hint below the input
  shows exactly which path will be created (or that the parent
  will be used).
- api.ts heaps.convert() signature accepts an optional
  subfolder_name field; the dialog sends it via mutationFn.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/services/api.ts

@@ -190,10 +190,16 @@ export const heaps = {
   },
 
   /** Convert a heap into a folder by moving (or copying) every member
-   *  photo into the target directory. */
+   *  photo into the target directory. Optionally creates a subfolder
+   *  inside the target by name. */
   convert: async (
     heapId: string,
-    body: { target_id: string; mode: 'move' | 'copy'; delete_heap: boolean }
+    body: {
+      target_id: string
+      mode: 'move' | 'copy'
+      delete_heap: boolean
+      subfolder_name?: string | null
+    }
   ) => {
     const response = await api.post(`/heaps/${heapId}/convert`, body)
     return response.data as {