Harden form auth and submission handling

drizzle/0011_unique_submission_user.sql

@@ -0,0 +1 @@
+CREATE UNIQUE INDEX "submission_form_user_discord_id_unique" ON "form"."submission" USING btree ("form_id","user_discord_id");

drizzle/meta/_journal.json

@@ -78,6 +78,13 @@
       "when": 1783089081925,
       "tag": "0010_rainy_speedball",
       "breakpoints": true
+    },
+    {
+      "idx": 11,
+      "version": "7",
+      "when": 1783613113217,
+      "tag": "0011_unique_submission_user",
+      "breakpoints": true
     }
   ]
-}
+}