fix: introduce ResourceAiSeat for fine-grained AI seat RBAC (#24613)

Fixes: https://github.com/coder/internal/issues/1444

coderd/database/migrations/000482_add_ai_seat_scopes.down.sql

@@ -0,0 +1,2 @@
+-- These enum values cannot be removed from PostgreSQL.
+-- This migration is a no-op placeholder for rollback safety.

coderd/database/migrations/000482_add_ai_seat_scopes.up.sql

@@ -0,0 +1,3 @@
+ALTER TYPE api_key_scope ADD VALUE IF NOT EXISTS 'ai_seat:*';
+ALTER TYPE api_key_scope ADD VALUE IF NOT EXISTS 'ai_seat:create';
+ALTER TYPE api_key_scope ADD VALUE IF NOT EXISTS 'ai_seat:read';