fix: store username on moderation_actions for retention safety

Add denormalized  column to moderation_actions so the
dashboard TARGET field survives message table purges. Backfills all
existing 217 rows. Changes: schema + autoDeleteManager + backend
repository query + migration 0017.

services/discord-gateway/src/shared/database/schema.ts

@@ -642,6 +642,7 @@ export const pgModerationActionsTable = pgTable(
     score: pgReal("score"), // 0..1 raw model score
     evidence: pgText("evidence"), // JSON array of short quoted snippets
     policy_version: pgText("policy_version"), // rules.ts policy version string
+    username: pgText("username"), // denormalized from messages table for retention safety
   },
   (table) => ({
     messageIdIdx: pgIndex("idx_moderation_actions_message_id").on(

services/discord-gateway/src/shared/database/schema/messages.ts

@@ -59,6 +59,7 @@ export const pgModerationActionsTable = pgTable(
     score: pgReal("score"), // 0..1 raw model score
     evidence: pgText("evidence"), // JSON array of short quoted snippets
     policy_version: pgText("policy_version"), // rules.ts policy version string
+    username: pgText("username"), // denormalized from messages table for retention safety
   },
   (table) => ({
     messageIdIdx: pgIndex("idx_moderation_actions_message_id").on(

services/discord-gateway/src/shared/moderation-types.ts

@@ -208,6 +208,7 @@ export interface ModerationAction {
   guild_id: string;
   action_type: ModerationActionType;
   reason: string | null;
+  username: string | null;
   executed_by: string | null;
   status: "pending" | "executed" | "failed";
   error: string | null;