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>

backend/app/routers/heaps.py

@@ -39,6 +39,10 @@ class HeapConvertBody(BaseModel):
     target_id: str  # folder id OR source root id
     mode: Literal['move', 'copy'] = 'move'
     delete_heap: bool = False
+    # Optional subfolder name to create inside the target. If provided, the
+    # actual destination is target_dir/subfolder_name (created if missing).
+    # Path separators and dot-segments are rejected.
+    subfolder_name: Optional[str] = None
 
 
 # ── Endpoints ─────────────────────────────────────────────────────────────
@@ -217,24 +221,53 @@ async def convert_heap_to_folder(
     source_root = sr_check.scalar_one_or_none()
 
     if source_root is not None:
-        target_dir = source_root.path
+        parent_dir = source_root.path
-        from app.tasks.scan import get_or_create_folder
+        parent_source_root_id = source_root.id
-        target_folder = await get_or_create_folder(db, target_dir, source_root.id)
     else:
         folder_check = await db.execute(
             select(Folder).where(Folder.id == body.target_id)
         )
-        target_folder = folder_check.scalar_one_or_none()
+        parent_folder = folder_check.scalar_one_or_none()
-        if target_folder is None:
+        if parent_folder is None:
             raise HTTPException(status_code=404, detail="Target folder not found")
-        target_dir = target_folder.path
+        parent_dir = parent_folder.path
+        parent_source_root_id = parent_folder.source_root_id
 
-    if not os.path.isdir(target_dir):
+    if not os.path.isdir(parent_dir):
         raise HTTPException(
             status_code=400,
-            detail=f"Target directory does not exist: {target_dir}",
+            detail=f"Target parent does not exist: {parent_dir}",
         )
 
+    # Resolve target_dir, creating an optional subfolder if requested.
+    if body.subfolder_name is not None:
+        sub = body.subfolder_name.strip()
+        if not sub:
+            raise HTTPException(status_code=400, detail="Subfolder name cannot be empty")
+        if '/' in sub or '\\' in sub or sub in ('.', '..'):
+            raise HTTPException(status_code=400, detail="Invalid subfolder name")
+        target_dir = os.path.join(parent_dir, sub)
+        if not os.path.exists(target_dir):
+            try:
+                os.makedirs(target_dir)
+            except OSError as e:
+                raise HTTPException(
+                    status_code=500,
+                    detail=f"Failed to create subfolder: {e}",
+                )
+        elif not os.path.isdir(target_dir):
+            raise HTTPException(
+                status_code=400,
+                detail=f"{target_dir} exists but is not a directory",
+            )
+    else:
+        target_dir = parent_dir
+
+    # Ensure a Folder row for the target, reusing the scanner helper so
+    # path normalization + dedupe stay consistent.
+    from app.tasks.scan import get_or_create_folder
+    target_folder = await get_or_create_folder(db, target_dir, parent_source_root_id)
+
     # Fetch the heap's photos via the join table.
     photo_result = await db.execute(
         select(Photo)

frontend/src/components/heaps/HeapConvertDialog.tsx

@@ -21,6 +21,7 @@ export function HeapConvertDialog({ heap, onClose }: HeapConvertDialogProps) {
   const [targetId, setTargetId] = useState('')
   const [mode, setMode] = useState<'move' | 'copy'>('move')
   const [deleteHeap, setDeleteHeap] = useState(false)
+  const [subfolderName, setSubfolderName] = useState('')
 
   const { data: foldersData } = useQuery({
     queryKey: ['folders'],
@@ -36,12 +37,15 @@ export function HeapConvertDialog({ heap, onClose }: HeapConvertDialogProps) {
     }
   }, [folders, targetId])
 
-  // Reset state on close.
+  // Reset state on close, prefill subfolder name when opened.
   useEffect(() => {
-    if (!heap) {
+    if (heap) {
+      setSubfolderName(heap.name)
+    } else {
       setTargetId('')
       setMode('move')
       setDeleteHeap(false)
+      setSubfolderName('')
     }
   }, [heap])
 
@@ -51,6 +55,9 @@ export function HeapConvertDialog({ heap, onClose }: HeapConvertDialogProps) {
         target_id: targetId,
         mode,
         delete_heap: deleteHeap,
+        // Empty subfolder = drop directly into the parent. Trim and only
+        // send if the user kept it populated.
+        subfolder_name: subfolderName.trim() || null,
       }),
     onSuccess: (data) => {
       const total = (data.moved ?? 0) + (data.copied ?? 0)
@@ -118,6 +125,25 @@ export function HeapConvertDialog({ heap, onClose }: HeapConvertDialogProps) {
           )}
         </div>
 
+        {/* Subfolder name */}
+        <div className="mb-4">
+          <label className="mb-1 block text-xs text-text-muted">
+            Subfolder name
+          </label>
+          <input
+            type="text"
+            value={subfolderName}
+            onChange={(e) => setSubfolderName(e.target.value)}
+            placeholder="(none — use parent directly)"
+            className="w-full rounded border border-border bg-bg px-2 py-1.5 text-sm text-text placeholder-text-faint focus:border-primary focus:outline-none"
+          />
+          <p className="mt-1 text-xs text-text-faint">
+            {subfolderName.trim() && targetFolder
+              ? `Will create ${targetFolder.path}/${subfolderName.trim()} if missing.`
+              : 'Photos go directly into the parent folder.'}
+          </p>
+        </div>
+
         {/* Mode toggle */}
         <div className="mb-4">
           <label className="mb-1 block text-xs text-text-muted">Mode</label>

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 {