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>

app/Http/Controllers/Api/HetznerController.php

@@ -121,7 +121,7 @@ class HetznerController extends Controller
 
             return response()->json($locations);
         } catch (\Throwable $e) {
-            return response()->json(['message' => 'Failed to fetch locations: '.$e->getMessage()], 500);
+            return response()->json(['message' => 'Failed to fetch Hetzner locations.'], 500);
         }
     }
 
@@ -242,7 +242,7 @@ class HetznerController extends Controller
 
             return response()->json($serverTypes);
         } catch (\Throwable $e) {
-            return response()->json(['message' => 'Failed to fetch server types: '.$e->getMessage()], 500);
+            return response()->json(['message' => 'Failed to fetch Hetzner server types.'], 500);
         }
     }
 
@@ -354,7 +354,7 @@ class HetznerController extends Controller
 
             return response()->json(array_values($filtered));
         } catch (\Throwable $e) {
-            return response()->json(['message' => 'Failed to fetch images: '.$e->getMessage()], 500);
+            return response()->json(['message' => 'Failed to fetch Hetzner images.'], 500);
         }
     }
 
@@ -450,7 +450,7 @@ class HetznerController extends Controller
 
             return response()->json($sshKeys);
         } catch (\Throwable $e) {
-            return response()->json(['message' => 'Failed to fetch SSH keys: '.$e->getMessage()], 500);
+            return response()->json(['message' => 'Failed to fetch Hetzner SSH keys.'], 500);
         }
     }
 
@@ -733,7 +733,7 @@ class HetznerController extends Controller
 
             return $response;
         } catch (\Throwable $e) {
-            return response()->json(['message' => 'Failed to create server: '.$e->getMessage()], 500);
+            return response()->json(['message' => 'Failed to create Hetzner server.'], 500);
         }
     }
 }