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/auth/dynamic_client_store.go

@@ -26,6 +26,12 @@ func (c *DynamicClient) HasRedirectURI(uri string) bool {
 	return false
 }
 
+// ClientStore is the interface implemented by both DynamicClientStore and PostgresClientStore.
+type ClientStore interface {
+	Register(name string, redirectURIs []string) (DynamicClient, error)
+	Lookup(clientID string) (DynamicClient, bool)
+}
+
 // DynamicClientStore holds dynamically registered OAuth clients in memory.
 type DynamicClientStore struct {
 	mu      sync.RWMutex