v0.1.2: bump checkpoint retention 30 -> 90

Each checkpoint is a few KB of JSON plus a tiny sidecar; even at 90
entries on a config with hundreds of endpoints the on-disk footprint
is negligible (worst case ~20 MB). With daily auto-checkpoints plus
on-save snapshots, 30 entries could fill in a couple weeks of
moderate use; 90 gives a comfortable ~3-month window.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Core/Storage/ConfigStore.cs

@@ -56,7 +56,7 @@ public sealed class ConfigStore
                         System.IO.Path.ChangeExtension(backupPath, ".meta.json"),
                         JsonSerializer.Serialize(sidecar, ConfigJson.Compact));
                 }
-                PruneBackups(backupsDir, retain: 30);
+                PruneBackups(backupsDir, retain: 90);
             }
             catch
             {