feat(library): tighten duplicates scope, drop in-app upload, polish loaders

- duplicates: restrict the /library/duplicates/groups query to photos whose
  folder path actually lives under an active SourceRoot in the user's
  settings. Nextcloud's "move to trash" flow was leaving .delete/purge-1
  Folder rows wired to the original source_root_id, leaking those entries
  into the Duplicates view as ghost paths that the user never opted into.
- discard: add a spinner to the "Delete N" and "Empty discard pile"
  buttons (and their confirm dialogs) while the destructive mutation is
  in flight, so the user gets immediate feedback for a slow operation.
- timeline: render a bottom-of-grid "Loading more photos…" indicator
  while usePhotosQuery's background cursor loop is still pulling pages.
  Backed by a tiny Zustand store the loop drives via a balanced
  start/stop (counter, not boolean, so rapid filter changes can't flip
  the flag false while a fresh loop is alive).
- remove client-side upload UI + /upload endpoint. Nextcloud is the
  authoritative ingress now; the duplicate path created confusion and
  the backend route is gone too.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -12,7 +12,7 @@ import os
 
 from app.config import settings
 from app.database import init_db
-from app.routers import photos, folders, heaps, tags, discard, library, search, auth, admin, sharing, upload, download, features, nextcloud, nc_webhook
+from app.routers import photos, folders, heaps, tags, discard, library, search, auth, admin, sharing, download, features, nextcloud, nc_webhook
 from app.services.scanner import start_initial_scan, bootstrap_default_source_root
 from app.services.cleanup import cleanup_data_integrity
 from app.services.nextcloud_dav import init_preview_client, close_preview_client
@@ -116,7 +116,6 @@ app.include_router(tags.router, prefix="/api/v1/tags", tags=["tags"])
 app.include_router(discard.router, prefix="/api/v1/discard", tags=["discard"])
 app.include_router(library.router, prefix="/api/v1/library", tags=["library"])
 app.include_router(search.router, prefix="/api/v1/photos/search", tags=["search"])
-app.include_router(upload.router, prefix="/api/v1/upload", tags=["upload"])
 app.include_router(download.router, prefix="/api/v1/download", tags=["download"])
 app.include_router(features.router, prefix="/api/v1/features", tags=["features"])
 app.include_router(nextcloud.router, prefix="/api/v1/nextcloud", tags=["nextcloud"])