docs(libraries): Work Order 3 batch 2 — 10 missing endpoints + array-response gate support

New frontmatter'd docs with handler-compatible shapes:
- get_library_types.md (public; []LibraryTypes)
- list_libraries.md (admin; {data: ListLibrariesRow})
- get_library_folders.md (admin; []LibraryFolders)
- list/get stats/resolve/delete processing issues (admin; family-lens 403s
  documented)
- scan_library.md (202 + 409 coalescing with active_job_id)
- get/update scan-settings (ScanSettingsResponse)

Gate improvements forced by this batch:
- Array responses: registered slice -> doc example must be a JSON array; key
  set compared against the element type
- RawMessage pins may now be arrays
- update scan-settings uses a RawMessage pin: handler always sets Message but
  the struct's omitempty would reject correct docs via zero-value marshal

Registry: sqlc models pinned directly (LibraryTypes, LibraryFolders,
ListLibrariesRow — json tags verified in the generated code).

docs/developer/api/libraries/list_libraries.md

@@ -0,0 +1,43 @@
+---
+method: GET
+path: /api/libraries
+auth: jwt+admin
+area: libraries
+---
+<!-- source: internal/handlers/library.go:102 LibraryHandler.ListLibraries -->
+
+# List Libraries (Admin)
+
+Lists **all** libraries regardless of per-user visibility. (Non-admin users
+see their subset via `GET /api/libraries/visibility`.)
+
+## Example response
+
+```json example-response
+{
+  "data": [
+    {
+      "id": "990e8400-e29b-41d4-a716-446655440000",
+      "name": "Manga",
+      "description": null,
+      "library_type_id": "880e8400-e29b-41d4-a716-446655440000",
+      "created_by_admin_id": "92413c3a-e1b8-4fff-b3f9-95c74478d19a",
+      "created_at": "2026-01-01T00:00:00Z",
+      "updated_at": "2026-01-01T00:00:00Z",
+      "type_name": "manga",
+      "type_description": null
+    }
+  ]
+}
+```
+
+Nullable fields (`description`, `type_description`) are JSON `null` when
+unset. `type_name`/`type_description` come from the joined library type.
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 401 | — | missing/invalid JWT |
+| 403 | — | not an admin |
+| 500 | `{"error": "..."}` | unexpected server error |