fix(discord-gateway): generate Drizzle migrations and copy to Docker image

Container crashed with 'Can't find meta/_journal.json file' because
Drizzle ORM migration files were never generated. Generated migrations
for all 10 schema tables and updated Dockerfile to copy drizzle/ to
/app/drizzle/ where the migrator expects them.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/discord-gateway/drizzle/migrations/meta/_journal.json

@@ -0,0 +1,13 @@
+{
+  "version": "7",
+  "dialect": "postgresql",
+  "entries": [
+    {
+      "idx": 0,
+      "version": "7",
+      "when": 1780329073891,
+      "tag": "0000_tricky_mysterio",
+      "breakpoints": true
+    }
+  ]
+}