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/update_scan_settings.md

@@ -0,0 +1,47 @@
+---
+method: PUT
+path: /api/libraries/scan-settings
+auth: jwt+admin
+area: libraries
+---
+<!-- source: internal/handlers/system_settings.go:210 SystemSettingsHandler.UpdateScanSettings -->
+
+# Update Scan Settings
+
+Sets the global scan poll interval and auto-scan flag, then reloads the
+settings registry so running code picks the new values up.
+
+## Example request
+
+```json example-request
+{
+  "scan_poll_interval_seconds": 3600,
+  "auto_scan_enabled": true
+}
+```
+
+## Example response
+
+```json example-response
+{
+  "scan_poll_interval_seconds": 3600,
+  "auto_scan_enabled": true,
+  "message": "scan settings updated successfully"
+}
+```
+
+## Request fields
+
+| Field | Type | Required | Notes |
+| ----- | ---- | -------- | ----- |
+| scan_poll_interval_seconds | number | yes | 1–3600 |
+| auto_scan_enabled | boolean | no | defaults false when omitted |
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "..."}` | validation failure |
+| 401 / 403 | — | missing JWT / not an admin |
+| 404 | `{"error": "system setting not found"}` | settings rows missing from DB |
+| 500 | `{"error": "..."}` | unexpected server error |