feat: watcher source-root resolution, folder rename, alt-drag copy

Three small phase-11 follow-ups in one commit since they all touch the
same surface area.

1. Watcher source-root resolution
   The watch_folders task previously called scan_folder.delay(parent_dir)
   when files arrived, with no source_root_id. scan_folder would then
   auto-create a fresh SourceRoot for that arbitrary subdir, polluting
   the source_root list. Now the watcher loads (path, id) pairs at
   startup, defines find_source_root_for() that walks the parent chain,
   and dispatches with the resolved id. Events under no known root are
   logged at debug and ignored instead of creating stale rows.

2. Folder rename via UI
   - Backend: PATCH /folders/{id} accepts { name } and updates the
     SourceRoot display label only. The on-disk path is controlled by
     the docker mount and intentionally not editable from the UI.
   - Frontend: double-click a folder row in the LeftSidebar to start
     editing; Enter or blur commits, Esc reverts. New renamingId /
     renameDraft local state and a renameMutation that invalidates
     ['folders']. The click handler ignores clicks while the row is
     in edit mode so it doesn't navigate.
   - api.ts: new sourceFolders.rename(id, name) helper.

3. Bulk copy via Alt-drag onto folder
   - Backend: new POST /photos/copy that mirrors /photos/move but uses
     shutil.copy2 and creates fresh Photo rows with is_duplicate=true.
     Name collisions are resolved by appending " (copy)", " (copy 2)",
     etc., up to 100 tries before erroring. Same target_id resolution
     as /move (folder id or source root id).
   - Frontend: photos.copy(ids, targetId) helper. LeftSidebar's
     handleDrop now takes a `copy` flag derived from e.altKey on the
     drop event; folder targets dispatch copyDropMutation when held,
     moveDropMutation otherwise. The drop-effect cursor flips to
     'copy' on dragover when Alt is pressed so the user gets visual
     confirmation. Discard target ignores the modifier.

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

backend/app/routers/folders.py

@@ -1,10 +1,11 @@
 """
-Folders API router. Source roots are config-driven (PHOTO_DIRS in .env →
-backend bootstrap on startup); this router only exposes read access and a
-manual rescan trigger. Adding/removing source roots happens by editing
-docker-compose.yml + .env and restarting the stack.
+Folders API router. Source roots themselves are config-driven (PHOTO_DIRS
+in .env → backend bootstrap on startup) — adding or removing one is a
+docker-compose change. The UI can read the list, trigger a manual rescan,
+and rename the display label, but it can't change the on-disk path.
 """
 from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 import os
@@ -14,6 +15,10 @@ from app.models import Folder, SourceRoot
 
 router = APIRouter()
 
+
+class FolderRename(BaseModel):
+    name: str
+
 @router.get("")
 async def get_folders(db: AsyncSession = Depends(get_db)):
     """Get all source folders"""
@@ -39,6 +44,28 @@ async def get_folders(db: AsyncSession = Depends(get_db)):
     
     return {"folders": folders_list}
 
+@router.patch("/{folder_id}")
+async def rename_folder(
+    folder_id: str,
+    body: FolderRename,
+    db: AsyncSession = Depends(get_db),
+):
+    """Rename a source root's display label. Does NOT touch the on-disk
+    path — that's controlled by the docker mount."""
+    name = (body.name or '').strip()
+    if not name:
+        raise HTTPException(status_code=400, detail="Name cannot be empty")
+
+    result = await db.execute(select(SourceRoot).where(SourceRoot.id == folder_id))
+    source_root = result.scalar_one_or_none()
+    if not source_root:
+        raise HTTPException(status_code=404, detail="Source folder not found")
+
+    source_root.name = name
+    await db.commit()
+    return {"id": source_root.id, "name": source_root.name, "path": source_root.path}
+
+
 @router.post("/{folder_id}/scan")
 async def scan_folder(folder_id: str, db: AsyncSession = Depends(get_db)):
     """Trigger manual re-scan of source root folder"""