feat(issues): retry a processing issue through the individual scanner

POST /api/libraries/:id/issues/:issueId/retry (admin, family-lens gated
via the existing gateIssue) re-runs the work that produced an open issue
— deliberately NOT a library-wide rescan. Two shapes mirror the two
ledger entry kinds:

- item-backed issues rescan their media item through the book-detail
  RescanMediaItem mechanism (self-resolving folders, multi-file
  audiobook routing, write-after-success guards) and then close the
  issue (RetryMediaItemIssue);
- row-less import_failed issues re-import the single file through the
  watcher's standalone processMediaFile path (RetryFailedImport: folders
  resolved RescanMediaItem-style so paths stay library-root-relative),
  whose create-path self-heal plus an explicit path resolve closes the
  issue even when the content was already imported by a prior attempt.

Failures return the REAL underlying error so the UI can show why the
issue still exists; a vanished file is a 409 (ErrRetryFileMissing);
already-resolved issues reject re-retry.

Docs + bruno mirrors regenerated for the new route (the generator's seq
renumbering touches three unrelated bruno files as fallout; openapi
gains the path and the date bump), and the docs schema registry gains
the retry entry so TestDocsMatchRoutes passes.

docs/developer/api/libraries/retry_processing_issue.md

@@ -0,0 +1,41 @@
+---
+method: POST
+path: /api/libraries/:id/issues/:issueId/retry
+auth: jwt+admin
+area: libraries
+---
+<!-- source: internal/handlers/processing_issues.go RetryProcessingIssue -->
+
+# Retry Processing Issue
+
+Re-runs the work that produced an open issue through the individual
+scanner — deliberately NOT a library-wide rescan. Two shapes:
+
+- **Item-backed issues** (media item exists): the item is rescanned via
+  the same mechanism as the book-detail Rescan action — self-resolving
+  folders, multi-file audiobook routing, write-after-success guards.
+- **Row-less `import_failed` issues** (no media item): the single file
+  is re-imported through the scanner's standalone per-file path.
+
+On success the issue is closed. On failure the response carries the REAL
+error (e.g. the underlying SQL failure), so the UI can show why the issue
+still exists. The issue's library must be in the requester's visible set.
+
+## Example response
+
+```json example-response
+{
+  "success": true,
+  "message": "File imported; issue resolved"
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "Invalid issue ID"}` | malformed path param |
+| 403 | — | issue's library not in requester's visible set |
+| 404 | `{"error": "Issue not found"}` | no such issue |
+| 409 | `{"error": "Issue is already resolved"}` / `{"error": "media file not found on disk: …"}` | nothing to retry, or the file vanished |
+| 500 | `{"error": "…"}` | the retried work failed — body carries the underlying error |