refactor(api): remove 5 deprecated list endpoints superseded by /resources

The normalized cursor-paginated /resources API is live and the bundled
frontend already uses it for favorites, recent, trash and grants. These five
deprecated old-format endpoints had no remaining frontend or protocol
consumers (the Nextcloud handlers call the service layer directly, not these
HTTP routes), so remove them for a uniform API and less duplicate listing
logic:

- GET /api/folders/{id}/contents          → use /api/folders/{id}/resources
- GET /api/folders/{id}/contents/paginated → use /api/folders/{id}/resources
- GET /api/favorites                       → use /api/favorites/resources
- GET /api/recent                          → use /api/recent/resources
- GET /api/trash                           → use /api/trash/resources

Removes the HTTP handlers, their routes, OpenAPI path registrations, and the
now-dead list_folder_contents{,_paginated}_impl helpers + unused imports. The
underlying service methods (favorites_service.get_favorites,
trash_service.get_trash_items, etc.) are KEPT — the Nextcloud OCS/trashbin
handlers depend on them.

Deliberately NOT removed: GET /api/folders/{id}/listing. It is still the Files
view's primary data path and offers ETag/304 conditional caching plus
one-shot favorite/share badge sets that /resources does not yet provide;
migrating it needs a separate parity pass on /resources first.

Updates the OpenAPI structure test and the two docs that referenced the
removed paths. `cargo clippy -D warnings` clean; 443 lib tests pass; OpenAPI
regenerates with the 5 paths gone and the /resources replacements present.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/plan/benchmake-and-performance-tracking.md

@@ -71,7 +71,7 @@ All scenarios load `baseline.json` at startup and set `thresholds` dynamically f
 
 **Endpoint contracts (confirmed by exploration):**
 - Login: `POST /api/auth/login` body `{username, password}` → `{access_token, ...}`.
-- Folder ops: `POST /api/folders`, `GET /api/folders/{id}/contents` (or `/contents/paginated` at high depth), `PUT /api/folders/{id}/move`, `POST /api/batch/folders/copy`, `DELETE /api/folders/{id}`. Handlers in `src/interfaces/api/handlers/folder_handler.rs` and `batch_handler.rs`.
+- Folder ops: `POST /api/folders`, `GET /api/folders/{id}/resources` (cursor-paginated; replaced the removed `/contents` + `/contents/paginated`), `PUT /api/folders/{id}/move`, `POST /api/batch/folders/copy`, `DELETE /api/folders/{id}`. Handlers in `src/interfaces/api/handlers/folder_handler.rs` and `batch_handler.rs`.
 - Grants: `POST /api/grants` (subject `{type, id|email}`, resource `{type, id}`, `role` or `permissions[]`), `GET /api/grants?resource_type=folder&resource_id=…`. Handler `src/interfaces/api/handlers/grant_handler.rs`.
 - Groups: `POST /api/groups` (admin), `POST /api/groups/{id}/members` body `{user_id}` or `{group_id}`. Handler `src/interfaces/api/handlers/subject_group_handler.rs`. Nesting limit is 8 — `--group-depth 3` is well inside.
 - Files: multipart `POST /api/files/upload` with fields `folder_id` + `file`.