feat(crypto): encrypt site/title/tags at rest too (CODE_AUDIT §1.3)

Extends the username-at-rest scheme to site, title and tags — the last
searchable metadata still stored cleartext. Same design: dedicated
<f>_enc/<f>_iv columns (AES-GCM under the vault key), decrypted at load into
e.<f>, so client-side search/sort/render/favicon/autofill-match are unchanged.
Full-strength random-IV AES-GCM (no searchable encryption) because search is
client-side.

Generalized the helpers over ENCRYPTED_META_FIELDS = [username, site, title,
tags]:
- withEncryptedUsername → withEncryptedMeta (encrypts all four, blanks
  cleartext) — wraps every POST/PUT body.
- decryptEntryUsernames → decryptEntryMeta (decrypts all four at load).
- migrateUsernamesAtRest → migrateMetadataAtRest (sweeps any field still
  cleartext, live + trash).
- doChangeMasterPassword re-encrypts all four under the new key.

Server (Entries + Auth + Database):
- Columns site_enc/iv, title_enc/iv, tags_enc/iv; GET emits them (new
  AddNullableField helper); POST/PUT/bulk read+persist (BindNullable helper);
  rotation UPDATE re-encrypts them.
- Removed the server "Site required" validation (site='' when encrypted — the
  client enforces it) at POST/PUT/bulk.
- ?q= server search neutralized (site+username ciphertext → LIKE useless; the
  frontend never sends ?search=).

Tests: merge assertions updated to decrypt site (encrypted on import). 65/65.

username was runtime-validated earlier; site/title/tags NOT yet compiled/
runtime-tested (Delphi) — large multi-handler change. Rebuild BuildAssets +
PMServer, then create/edit/dup/move/tag/import/rotate and verify the DB shows
no cleartext site/title/tags (and the app still renders/searches).

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

js/app.import.js

@@ -456,7 +456,7 @@ async function encryptImportEntry(plain) {
         : (plain.tags || '');
     // Encrypt the username at rest (username_enc/username_iv, cleartext blanked)
     // via the shared choke point in app.js — same as the interactive save path.
-    return await withEncryptedUsername({
+    return await withEncryptedMeta({
         uuid:               plain.uuid || '',
         site:               plain.site,
         title:              plain.title || '',