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/scan_library.md

@@ -0,0 +1,35 @@
+---
+method: POST
+path: /api/libraries/:id/scan
+auth: jwt+admin
+area: libraries
+---
+<!-- source: internal/router/library.go:43 (wraps handlers/scanner.go:34 Handler.ScanLibrary) -->
+
+# Scan Library
+
+Enqueues a full scan of one library's configured folders. The library id comes
+from the path — no request body is needed (the generic `POST /api/scanner/scan`
+variant is the one that takes `folder_paths`).
+
+Scans are **coalesced**: if a scan for this library is already pending or
+running you get **409** with the active job id instead of a second scan.
+
+## Example response (202 Accepted)
+
+```json example-response
+{
+  "message": "scan job enqueued",
+  "job_id": "dd0e8400-e29b-41d4-a716-446655440000",
+  "status": "pending"
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid library id"}` / `{"error": "library has no folders configured"}` | malformed id / empty library |
+| 404 | `{"error": "library not found or has no folders"}` | unknown library |
+| 409 | `{"error": "a scan for this library is already pending or running", "active_job_id": "..."}` | scan coalescing |
+| 500 | `{"error": "failed to enqueue scan job: ..."}` | queue failure |