fix: prevent duplicate DMs at creation time, clean up corrupted read states

Replace the unreliable client-side DM dedup loop in populateFromReady with
a creation-time guard (findExistingDmForUser) that checks all instances
before opening a new DM. Guards added to FriendsPage, NewDmModal, and
UserProfilePopout.

Also fixes: corrupted read_states from temp_ optimistic message IDs (server
migration + client-side validation), federation-aware closeDm/addDmMember
API routing, isSelf-based DM member filtering in sidebar/header, and WS
event error isolation.

packages/server/src/db/migrate.ts

@@ -138,6 +138,9 @@ export function runMigrations(db: Database.Database): void {
   // ─── Admin flag: ensure at least one admin exists (first registered user) ──
   migrateFirstAdmin(db);
 
+  // ─── Clean up corrupted read_states (temp_ IDs leaked from optimistic messages) ─
+  migrateCorruptedReadStates(db);
+
   console.log('Migrations complete.');
 }
 
@@ -236,6 +239,16 @@ function migrateEveryoneRoles(db: Database.Database): void {
   }
 }
 
+/** Delete corrupted read_states rows where last_read_message_id is not a valid snowflake (numeric string) */
+function migrateCorruptedReadStates(db: Database.Database): void {
+  const deleted = db.prepare(
+    "DELETE FROM read_states WHERE last_read_message_id NOT GLOB '[0-9]*' OR last_read_message_id GLOB '*[^0-9]*'"
+  ).run();
+  if (deleted.changes > 0) {
+    console.log(`Migrating: Cleaned up ${deleted.changes} corrupted read_states rows`);
+  }
+}
+
 /**
  * Rename non-namespaced replicated users: e.g. "test" → "test@nova.ddns.net"
  * Frees plain usernames for native user creation and makes all federated users