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/Privacy/PrivacyDefaults.cs

@@ -41,4 +41,47 @@ internal static class PrivacyDefaults
         ChatType.ExtraChatLinkshell7,
         ChatType.ExtraChatLinkshell8,
     };
+
+    // Default retention windows per channel (in days). Channels not listed
+    // here fall back to Configuration.RetentionDefaultDays. Reflects the
+    // design spec: Tells 365, own-conversation channels 90, everything else
+    // shorter via the global default.
+    internal static readonly IReadOnlyDictionary<ChatType, int> DefaultRetentionDays = new Dictionary<ChatType, int>
+    {
+        [ChatType.TellIncoming] = 365,
+        [ChatType.TellOutgoing] = 365,
+
+        [ChatType.Party] = 90,
+        [ChatType.CrossParty] = 90,
+        [ChatType.Alliance] = 90,
+        [ChatType.PvpTeam] = 90,
+        [ChatType.FreeCompany] = 90,
+
+        [ChatType.Linkshell1] = 90,
+        [ChatType.Linkshell2] = 90,
+        [ChatType.Linkshell3] = 90,
+        [ChatType.Linkshell4] = 90,
+        [ChatType.Linkshell5] = 90,
+        [ChatType.Linkshell6] = 90,
+        [ChatType.Linkshell7] = 90,
+        [ChatType.Linkshell8] = 90,
+
+        [ChatType.CrossLinkshell1] = 90,
+        [ChatType.CrossLinkshell2] = 90,
+        [ChatType.CrossLinkshell3] = 90,
+        [ChatType.CrossLinkshell4] = 90,
+        [ChatType.CrossLinkshell5] = 90,
+        [ChatType.CrossLinkshell6] = 90,
+        [ChatType.CrossLinkshell7] = 90,
+        [ChatType.CrossLinkshell8] = 90,
+
+        [ChatType.ExtraChatLinkshell1] = 90,
+        [ChatType.ExtraChatLinkshell2] = 90,
+        [ChatType.ExtraChatLinkshell3] = 90,
+        [ChatType.ExtraChatLinkshell4] = 90,
+        [ChatType.ExtraChatLinkshell5] = 90,
+        [ChatType.ExtraChatLinkshell6] = 90,
+        [ChatType.ExtraChatLinkshell7] = 90,
+        [ChatType.ExtraChatLinkshell8] = 90,
+    };
 }