refactor(api): remove 5 dead routes + stale deprecation markers

An audit (backend /api routes vs SvelteKit frontend usage, adversarially
verified across the whole repo) found these 5 routes have ZERO callers — no
frontend, no test, no protocol layer, no internal caller — and are superseded:

- GET  /api/folders/paginated            no-op duplicate of GET /api/folders
                                         (discards the page arg); superseded by
                                         the cursor-paginated /{id}/resources.
- POST /api/dedup/upload                 superseded by /api/files/upload, which
                                         does the identical CDC dedup ingest.
- POST /api/people/{id}/hide             the hide-person toggle was never built
                                         into the UI (is_hidden never read).
- GET  /api/admin/settings/general       never called anywhere.
- GET  /api/admin/settings/registration  only the PUT is used; the GET had no
                                         caller (PUT kept).

Removes each route, its handler + _impl, the now-orphaned DedupUploadResponse
DTO + its two serialization tests, the set_hidden service method (only caller
was hide_person), and the OpenAPI path/schema registrations.

Also cleans 4 stale markers: two #[allow(deprecated)] that no longer suppress
anything (zero #[deprecated] remain), the "Legacy folder endpoints (contents,
listing)" comment (both already removed), and a "Re-export AppError for backward
compatibility" comment describing a re-export that doesn't exist.

Net -323 lines. The 31 other unused-by-frontend routes (device-code auth,
CardDAV contact-groups, people/photos & music WIP, dedup/admin debug, i18n,
openapi.json) are intentional surface and were left untouched.

cargo clippy --all-features --all-targets -D warnings: clean. cargo test: 446 passed.

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

src/interfaces/api/handlers/folder_handler.rs

@@ -17,7 +17,6 @@ use crate::application::dtos::folder_dto::{
     ListResourcesOptions, MoveFolderDto, RenameFolderDto,
 };
 use crate::application::dtos::grant_dto::{ResourceContentDto, ResourceTypeDto};
-use crate::application::dtos::pagination::PaginationRequestDto;
 use crate::application::ports::folder_ports::FolderUseCase;
 use crate::application::ports::trash_ports::TrashUseCase;
 use crate::application::services::folder_service::FolderService;
@@ -107,16 +106,6 @@ impl FolderHandler {
         Self::list_folders_scoped(service, None, &auth_user).await
     }
 
-    /// Lists root folders with pagination.
-    /// Scoped to the authenticated user — only returns folders owned by this user.
-    pub(super) async fn list_root_folders_paginated_impl(
-        State(service): State<AppState>,
-        auth_user: AuthUser,
-        _pagination: Query<PaginationRequestDto>,
-    ) -> axum::response::Response {
-        Self::list_folders_scoped(service, None, &auth_user).await
-    }
-
     /// Internal helper: lists folders scoped to the authenticated user.
     /// Uses `list_folders_for_owner` — the DB query filters by `user_id`,
     /// so no data from other users ever leaves the database.
@@ -378,24 +367,6 @@ pub async fn list_root_folders(
     FolderHandler::list_root_folders_impl(state, auth_user).await
 }
 
-#[utoipa::path(
-    get,
-    path = "/api/folders/paginated",
-    params(PaginationRequestDto),
-    responses(
-        (status = 200, description = "Paginated list of root folders"),
-    ),
-    security(("bearerAuth" = [])),
-    tag = "folders"
-)]
-pub async fn list_root_folders_paginated(
-    state: State<AppState>,
-    auth_user: AuthUser,
-    pagination: Query<PaginationRequestDto>,
-) -> axum::response::Response {
-    FolderHandler::list_root_folders_paginated_impl(state, auth_user, pagination).await
-}
-
 #[utoipa::path(
     put,
     path = "/api/folders/{id}/rename",