refactor(api): return generic error messages for upstream and storage failures

Replace exception text in 5xx JSON responses with stable, action-specific
messages so API consumers get a consistent payload regardless of which
underlying client (Guzzle, PDO, filesystem) raised the exception. The
previous responses concatenated the raw upstream error, which produced
inconsistent messages and unnecessary noise for clients trying to parse
errors programmatically.

Touched endpoints:
- GET /api/v1/hetzner/{locations,server-types,images,ssh-keys}
- POST /api/v1/servers/hetzner
- DELETE /api/v1/databases/{uuid}/backups/{uuid}
- DELETE /api/v1/databases/{uuid}/backups/{uuid}/executions/{uuid}
- /download/backup/{uuid}

The RateLimitException branch and AuthenticationException flow keep their
existing curated messages.

Adds Pest coverage for the four Hetzner GET endpoints to lock the response
shape on upstream failure.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

routes/web.php

@@ -391,7 +391,7 @@ Route::middleware(['auth'])->group(function () {
                 'Content-Disposition' => 'attachment; filename="'.basename($filename).'"',
             ]);
         } catch (Throwable $e) {
-            return response()->json(['message' => $e->getMessage()], 500);
+            return response()->json(['message' => 'Failed to download backup.'], 500);
         }
     })->name('download.backup');