feat(db): add oauth_clients table for dynamic client registration

* Introduced oauth_clients table with fields for client_id, client_name, redirect_uris, and created_at.
* Updated agent_persona_parts, agent_persona_skills, agent_persona_guardrails, agent_persona_traits, and arc_stage_parts tables to use unique constraints instead of primary keys for composite indexes.

internal/store/db.go

@@ -131,3 +131,10 @@ func (db *DB) Bun() *bun.DB {
 	}
 	return db.bun
 }
+
+func (db *DB) Pool() *pgxpool.Pool {
+	if db == nil {
+		return nil
+	}
+	return db.pool
+}