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_scan_settings.md

@@ -0,0 +1,30 @@
+---
+method: GET
+path: /api/libraries/scan-settings
+auth: jwt+admin
+area: libraries
+---
+<!-- source: internal/handlers/system_settings.go:253 SystemSettingsHandler.GetScanSettings -->
+
+# Get Scan Settings
+
+Returns the global scan poll interval and auto-scan flag (system settings,
+admin-scoped). Defaults to 1800s / enabled when unset.
+
+## Example response
+
+```json example-response
+{
+  "scan_poll_interval_seconds": 1800,
+  "auto_scan_enabled": true
+}
+```
+
+`message` appears only in the update response (`omitempty`).
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 401 / 403 | — | missing JWT / not an admin |
+| 500 | `{"error": "..."}` | unexpected server error |