feat(crypto): encrypt username at rest (CODE_AUDIT §1.3)

username is no longer stored cleartext. New columns username_enc/username_iv
(AES-GCM under the vault key, same as encrypted_password). Search/sort/render
stay client-side, so the field is decrypted at loadEntries into e.username in
memory — everything downstream is unchanged. Full-strength random-IV AES-GCM
(no searchable/deterministic encryption) precisely because search is
client-side.

Server (PM.Handler.Entries / .Auth / PM.Database):
- Schema: vault_entries.username_enc, username_iv.
- GET returns them; POST/PUT/bulk-import read + persist them; master-pw
  rotation re-encrypts them under the new key (UPDATE + loop).
- ?q= server search drops `username LIKE` (ciphertext won't match; frontend
  searches client-side anyway).

Client (app.js / app.import.js):
- loadEntries/loadTrash decrypt username_enc → e.username (fallback to
  cleartext for un-migrated rows).
- withEncryptedUsername(obj): write choke point — encrypts obj.username into
  username_enc/username_iv and blanks the cleartext. Wraps every POST/PUT
  body: saveEntry, soSave, duplicateEntry, moveEntryToFolder, addTagToEntry,
  batchMove/AddTag, encryptImportEntry (import + sync-apply).
- doChangeMasterPassword re-encrypts username under the new key.
- migrateUsernamesAtRest(): one-time sweep at enterApp, PUT-re-ships rows that
  still carry cleartext username so the DB gets scrubbed (bumps updated_at
  once; plaintext unchanged so devices converge).

site/title/tags stay cleartext (same pattern later — see memory note). +1
merge test (username encrypted on import). 65/65.

NOT compiled/tested at runtime (Delphi) — large multi-handler change; rebuild
BuildAssets + PMServer and test create/edit/rotate/import/sync + verify the
DB shows no cleartext username.

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

delphi-backend/Source/PM.Database.pas

@@ -353,6 +353,14 @@ begin
   // columns as opaque ciphertext + IV.
   AddColumnIfMissing('vault_entries', 'custom_fields',     'TEXT');
   AddColumnIfMissing('vault_entries', 'custom_fields_iv',  'TEXT');
+  // Encrypted username (AES-GCM under the vault key, same as encrypted_password).
+  // Metadata-at-rest hardening (CODE_AUDIT §1.3): the cleartext `username`
+  // column is phased out — new writes store '' there and put the ciphertext
+  // here. Old rows keep cleartext until the client migration sweep re-encrypts
+  // them at unlock. Search/sort stay client-side on the decrypted in-memory
+  // value, so no server-side change to those. NULL = not yet encrypted.
+  AddColumnIfMissing('vault_entries', 'username_enc',      'TEXT');
+  AddColumnIfMissing('vault_entries', 'username_iv',       'TEXT');
   // Cached favicon as a base64 data URI (e.g. "data:image/png;base64,...").
   // Fetched on demand by the Delphi favicon proxy when the user opts in.
   // NULL = no icon cached → JS falls back to the first-letter avatar.