Add Discord outbound posting.

internal/store/migrate_posts_test.go

@@ -81,6 +81,12 @@ CREATE TABLE users (
 	if err := migratePostImages(ctx, conn); err != nil {
 		t.Fatalf("post images migration is not idempotent: %v", err)
 	}
+	if err := migrateDiscordPostLinks(ctx, conn); err != nil {
+		t.Fatal(err)
+	}
+	if err := migrateDiscordPostLinks(ctx, conn); err != nil {
+		t.Fatalf("discord post links migration is not idempotent: %v", err)
+	}
 	if _, err := conn.ExecContext(ctx, `
 INSERT INTO users (id, name, role)
 VALUES ('homeowner', 'Home Owner', 'user'), ('plumber', 'The Plumber', 'admin');
@@ -143,6 +149,42 @@ VALUES ('homeowner', 'root-1', 1);`); err != nil {
 	}); err == nil {
 		t.Fatal("fifth image position unexpectedly succeeded")
 	}
+	if err := imageQueries.UpsertDiscordPostLink(ctx, sqlc.UpsertDiscordPostLinkParams{
+		PostID:           "root-1",
+		DiscordMessageID: "msg-root",
+		DiscordThreadID:  "thread-root",
+		CreatedAt:        "2026-08-26T08:00:00Z",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := imageQueries.UpsertDiscordPostLink(ctx, sqlc.UpsertDiscordPostLinkParams{
+		PostID:           "reply-1",
+		DiscordMessageID: "msg-reply",
+		DiscordThreadID:  "",
+		CreatedAt:        "2026-08-26T09:00:00Z",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	rootLink, err := imageQueries.GetDiscordPostLinkByPostID(ctx, "root-1")
+	if err != nil || rootLink.DiscordMessageID != "msg-root" || rootLink.DiscordThreadID != "thread-root" {
+		t.Fatalf("root discord link = %+v, %v", rootLink, err)
+	}
+	threadLink, err := imageQueries.GetDiscordPostLinkByThreadID(ctx, "thread-root")
+	if err != nil || threadLink.PostID != "root-1" {
+		t.Fatalf("thread discord link = %+v, %v", threadLink, err)
+	}
+	if err := imageQueries.UpsertDiscordPostLink(ctx, sqlc.UpsertDiscordPostLinkParams{
+		PostID:           "reply-1",
+		DiscordMessageID: "msg-reply-2",
+		DiscordThreadID:  "",
+		CreatedAt:        "2026-08-26T09:01:00Z",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	replyLink, err := imageQueries.GetDiscordPostLinkByPostID(ctx, "reply-1")
+	if err != nil || replyLink.DiscordMessageID != "msg-reply-2" || replyLink.DiscordThreadID != "" {
+		t.Fatalf("reply upsert = %+v, %v", replyLink, err)
+	}
 	var postVoteIndexCount int
 	if err := conn.QueryRowContext(ctx, `
 SELECT count(*)