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_processing_issues.md

@@ -0,0 +1,40 @@
+---
+method: GET
+path: /api/libraries/:id/issues/list
+auth: jwt+admin
+area: libraries
+---
+<!-- source: internal/handlers/processing_issues.go:83 ProcessingIssuesHandler.ListProcessingIssues -->
+
+# List Processing Issues
+
+Lists scan/processing issues for a library (import failures, metadata
+problems). Row-less import failures have no media item — their
+`media_item_id` is `""` and `title` falls back to the file path.
+
+## Example response
+
+```json example-response
+[
+  {
+    "id": "bb0e8400-e29b-41d4-a716-446655440000",
+    "media_item_id": "cc0e8400-e29b-41d4-a716-446655440000",
+    "title": "1984",
+    "file_path": "/books/1984.epub",
+    "format_group": "text",
+    "library_type_name": "ebooks",
+    "issue_type": "import_failed",
+    "issue_description": "unsupported format",
+    "severity": "error",
+    "created_at": "2026-10-24T13:57:01Z"
+  }
+]
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 403 | — | library not in requester's visible set (family lens) |
+| 404 | — | no such library |
+| 500 | `{"error": "Failed to fetch issues"}` | unexpected server error |