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.

schema/oauth_clients.dbml

@@ -0,0 +1,8 @@
+// OAuth 2.0 Dynamic Client Registration (RFC 7591)
+Table oauth_clients {
+  id            bigserial    [pk]
+  client_id     text         [unique, not null]
+  client_name   text         [not null, default: '']
+  redirect_uris "text[]"     [not null, default: `'{}'`]
+  created_at    timestamptz  [not null, default: `now()`]
+}