feat(nextcloud): per-user Nextcloud library integration

Lets each mule-image user (matched via OIDC preferred_username,
overridable in Settings) browse their Nextcloud files/ tree from the
mule-image UI and register subfolders as per-user SourceRoots. Reads
stay direct on the bind-mounted /nextcloud-users path; mutations
(upload, delete, rename, move within NC) dispatch through Nextcloud
WebDAV so oc_filecache, trashbin, comments, and desktop-sync clients
stay coherent.

Backend:
- users.nextcloud_username + nextcloud_app_password_enc (Fernet at rest,
  key derived from SECRET_KEY) — alembic 0016
- services/nextcloud_dav.py: minimal WebDAV client (PUT, MKCOL, DELETE,
  MOVE) with HTTP Basic auth via the per-user app password
- routers/nextcloud.py: GET /browse, /whoami, GET/POST/DELETE
  /source-roots (path-scoped to current_user.nextcloud_username with
  realpath traversal guard)
- PATCH /api/v1/auth/me to update nextcloud_username and app password
- OIDC callback defaults nextcloud_username from preferred_username on
  first login; backfill on existing users; never overwrites a manual
  override
- routers/upload.py: stream upload to NamedTemporaryFile, then PUT to
  WebDAV (with MKCOL chain) when destination is NC-rooted; existing
  Photo row creation runs unchanged
- routers/discard.py empty-trash: WebDAV DELETE for NC files
- routers/photos.py rename + move: WebDAV MOVE for NC paths;
  cross-system move/copy returns a clean error
- routers/folders.py rename + create + permanent-delete: dispatch via
  WebDAV when targeting NC-rooted paths

Frontend:
- AuthUser carries nextcloud_username + has_nextcloud_app_password
- services/api.ts: nextcloud + account namespaces
- components/dialogs/NextcloudFolderPicker.tsx: lazy tree browser, name
  + submit -> POST /source-roots
- SettingsDialog: new "Nextcloud library" card with username override +
  validate, app-password input, list/remove of NC libraries, and the
  picker entry point

docker-compose.yml: NEXTCLOUD_USERS_HOST_PATH bind to /nextcloud-users
on backend + 3 workers; NEXTCLOUD_USERS_ROOT + NEXTCLOUD_BASE_URL env.

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
+from app.routers import photos, folders, heaps, tags, discard, library, search, auth, admin, sharing, upload, download, features, nextcloud
 from app.services.scanner import start_initial_scan, bootstrap_default_source_root
 from app.services.cleanup import cleanup_data_integrity
 
@@ -112,6 +112,7 @@ 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"])
 
 @app.get("/")
 async def root():