Add per-channel message retention with daily background sweep

Privacy filter trimmed history "by what" — this adds the time axis.
Each ChatType gets its own retention window in days; channels
without an explicit override fall back to a configurable global
default. The master switch defaults to OFF: the plugin never
deletes history without explicit user consent.

MessageStore.DeleteByRetentionPolicy builds an OR'd WHERE clause
over (ChatType = X AND Date < cutoff_X) plus a NOT IN catch-all
for the global default, hard-deletes matches, and only runs VACUUM
when something was actually removed.

Plugin.RunRetentionSweepIfDue runs at most once per 24 hours on a
background thread (off the load path) and persists the timestamp
so subsequent restarts skip the sweep until enough time has
passed. The Privacy tab gains a retention section with the master
switch, default-days input, per-channel override tree, reset
buttons, and a Ctrl+Shift "apply now" action that mirrors the
auto-sweep but on demand.

Spec defaults: Tells 365 days, own-conversation channels (Party,
Cross-Party, Alliance, PvP Team, FC, Linkshells 1-8, Cross-World
Linkshells 1-8, ExtraChat 1-8) 90 days, fallback 30 days.

ChatTwo/Configuration.cs

@@ -54,6 +54,21 @@ public class Configuration : IPluginConfiguration
         return PrivacyPersistUnknownChannels;
     }
 
+    // Hellion Chat — Message retention (GDPR data minimization, time axis).
+    // Master switch defaults to false; the plugin will not delete history
+    // until the user explicitly opts in.
+    public bool RetentionEnabled;
+    public int RetentionDefaultDays = 30;
+    public Dictionary<ChatType, int> RetentionPerChannelDays = [];
+    public DateTimeOffset RetentionLastRunAt = DateTimeOffset.MinValue;
+
+    public int GetRetentionDays(ChatType type)
+    {
+        if (RetentionPerChannelDays.TryGetValue(type, out var userOverride))
+            return userOverride;
+        return RetentionDefaultDays;
+    }
+
     public bool HideChat = true;
     public bool HideDuringCutscenes = true;
     public bool HideWhenNotLoggedIn = true;
@@ -216,6 +231,11 @@ public class Configuration : IPluginConfiguration
         PrivacyFilterEnabled = other.PrivacyFilterEnabled;
         PrivacyPersistChannels = [..other.PrivacyPersistChannels];
         PrivacyPersistUnknownChannels = other.PrivacyPersistUnknownChannels;
+
+        RetentionEnabled = other.RetentionEnabled;
+        RetentionDefaultDays = other.RetentionDefaultDays;
+        RetentionPerChannelDays = other.RetentionPerChannelDays.ToDictionary(p => p.Key, p => p.Value);
+        RetentionLastRunAt = other.RetentionLastRunAt;
     }
 }