Add endpoint for querying systems by Discord snowflake

PluralKit.API/Controllers/AccountController.cs

@@ -0,0 +1,27 @@
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc;
+
+namespace PluralKit.API.Controllers
+{
+    [ApiController]
+    [Route("a")]
+    [Route("v1/a")]
+    public class AccountController: ControllerBase
+    {
+        private SystemStore _systems;
+
+        public AccountController(SystemStore systems)
+        {
+            _systems = systems;
+        }
+
+        [HttpGet("{aid}")]
+        public async Task<ActionResult<PKSystem>> GetSystemByAccount(ulong aid)
+        {
+            var system = await _systems.GetByAccount(aid);
+            if (system == null) return NotFound("Account not found.");
+            
+            return Ok(system);
+        }
+    }
+}