fix(chat): resolve merged-away user ids so DMs to merged contacts don't vanish

Root cause of 'messages to some contacts disappear (gone after reopen)': the #2
account merge deletes the merged-away user row. Any lingering reference to that old
id — a cached contact, an in-flight DM — then saved against a dead recipient / 404'd
on thread fetch, so messages silently vanished.

- New user_aliases table records old_id -> survivor on every merge (mergeInto).
- users.resolve(id) follows the redirect.
- DM send (recipient), thread fetch (with), and read now resolve() the peer id, so a
  stale id transparently routes to the surviving account.

Fixes future merges fully. Contacts merged BEFORE this (no alias recorded) may need a
one-off data check.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/db.js

@@ -225,6 +225,16 @@ try { db.exec("ALTER TABLE users ADD COLUMN status TEXT NOT NULL DEFAULT 'active
 // Provisioning matches on it to keep one Biz Connect account per person (#2 account merge).
 try { db.exec('ALTER TABLE users ADD COLUMN bizgaze_user_id TEXT'); } catch (e) { /* exists */ }
 try { db.exec('CREATE INDEX IF NOT EXISTS idx_users_bizgaze_user_id ON users(bizgaze_user_id)'); } catch (e) { /* exists */ }
+// When two accounts merge (#2), the merged-away row is deleted. This records old_id -> survivor so
+// any lingering reference to the old id (a cached contact, an in-flight DM) resolves to the survivor
+// instead of hitting a deleted user (which made messages to merged contacts silently vanish).
+db.exec(`
+CREATE TABLE IF NOT EXISTS user_aliases (
+  old_id TEXT PRIMARY KEY,
+  user_id TEXT NOT NULL,
+  team_id TEXT,
+  created_at INTEGER NOT NULL
+)`);
 db.exec(`
 CREATE TABLE IF NOT EXISTS polls (
   id TEXT PRIMARY KEY,