fix(db): add migration for corrected_moderations table

Table was defined in Drizzle schema at schema.ts:425 but no migration
existed. Runtime queries from text-cache-store fail with:
  relation "corrected_moderations" does not exist

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/discord-gateway/drizzle/migrations/0003_add_corrected_moderations.sql

@@ -0,0 +1,12 @@
+CREATE TABLE "corrected_moderations" (
+	"id" text PRIMARY KEY NOT NULL,
+	"message_id" text NOT NULL,
+	"original_flags" text NOT NULL,
+	"corrected_flags" text NOT NULL,
+	"correction_notes" text,
+	"content_snippet" text NOT NULL,
+	"created_at" bigint NOT NULL
+);
+--> statement-breakpoint
+CREATE INDEX "idx_corrected_moderations_created_at" ON "corrected_moderations" USING btree ("created_at");--> statement-breakpoint
+CREATE INDEX "idx_corrected_moderations_message_id" ON "corrected_moderations" USING btree ("message_id");