feat: add assigneeId/assigneeName to tasks schema and API

- Added assignee_id and assignee_name columns to tasks table
- PATCH /api/tasks/:id now accepts assigneeId and assigneeName
- db:push on startup will auto-migrate the columns

backend/drizzle/meta/_journal.json

@@ -0,0 +1,13 @@
+{
+  "version": "7",
+  "dialect": "postgresql",
+  "entries": [
+    {
+      "idx": 0,
+      "version": "7",
+      "when": 1769658956087,
+      "tag": "0000_grey_starhawk",
+      "breakpoints": true
+    }
+  ]
+}