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/get_library_folders.md

@@ -0,0 +1,32 @@
+---
+method: GET
+path: /api/libraries/:id/folders
+auth: jwt+admin
+area: libraries
+---
+<!-- source: internal/handlers/library.go:216 LibraryHandler.GetLibraryFolders -->
+
+# Get Library Folders
+
+Lists the filesystem folders configured for a library.
+
+## Example response
+
+```json example-response
+[
+  {
+    "id": "aa0e8400-e29b-41d4-a716-446655440000",
+    "library_id": "990e8400-e29b-41d4-a716-446655440000",
+    "folder_path": "/books/manga",
+    "created_at": "2026-01-01T00:00:00Z"
+  }
+]
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid library id"}` | malformed id |
+| 401 / 403 | — | missing JWT / not an admin |
+| 500 | `{"error": "..."}` | unexpected server error |