Add Discord outbound posting (#14)

Stacks on #13. Adds discord_post_links and a subscriber that posts roots into a public thread, replies into that thread, and edits the linked Discord message. Unset Discord env leaves the site unchanged.

Reviewed-on: #14
Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/store/migrate.go

@@ -139,6 +139,25 @@ CREATE TABLE IF NOT EXISTS post_images (
 	return nil
 }
 
+func migrateDiscordPostLinks(ctx context.Context, exec execContext) error {
+	if _, err := exec.ExecContext(ctx, `
+CREATE TABLE IF NOT EXISTS discord_post_links (
+  post_id TEXT PRIMARY KEY REFERENCES posts(id) ON DELETE CASCADE,
+  discord_message_id TEXT NOT NULL UNIQUE,
+  discord_thread_id TEXT NOT NULL DEFAULT '',
+  created_at TEXT NOT NULL
+)`); err != nil {
+		return fmt.Errorf("create discord_post_links: %w", err)
+	}
+	if _, err := exec.ExecContext(ctx, `
+CREATE UNIQUE INDEX IF NOT EXISTS discord_post_links_thread_uidx
+  ON discord_post_links (discord_thread_id)
+  WHERE discord_thread_id <> ''`); err != nil {
+		return fmt.Errorf("discord_post_links_thread_uidx: %w", err)
+	}
+	return nil
+}
+
 func migratePostDate(ctx context.Context, exec execContext) error {
 	steps := []struct {
 		name string
@@ -331,6 +350,7 @@ CREATE TABLE IF NOT EXISTS schema_migrations (
 		{"008_post_author_index", migratePostAuthorIndex},
 		{"009_drop_legacy_post_tables", migrateDropLegacyPostTables},
 		{"010_post_images", migratePostImages},
+		{"011_discord_post_links", migrateDiscordPostLinks},
 	}
 	for _, m := range migrations {
 		if applied[m.version] {