[glm-grade=A] fix(config): teach schema KNOWN_KEYS the licenseBackup/_version writer keys (DC-091)

Every startup logged two false-positive 'Unknown config key — possible typo?'
warns: licenseBackup (written by src/managers/license-manager.js:510 activation
persistence) and _version (stamped by src/config/migrations.js). Both are
first-party writers the validator was never taught about (DC-091).

- config-schema.js: add both keys to KNOWN_KEYS with a source-of-writes comment
- config-schema.test.js (new): 5 regression tests — live production config keyset
  validates with zero unknown-key warnings, writer keys never warn, genuine
  typos still warn (exact string), license/licenseBackup sync guard, _version
  recognized at every migration value

Verified: full jest suite 111 suites / 2621 tests green (baseline 110/2616).
Warns reproduced in live container logs 2026-08-22T23:53:54Z; live config.json
contains both keys (licenseBackup activation, _version 2).

Judge: GLM-5.3 cold read via delegate_task (deleg_30e52384, 36s) — grade A, ship.
Verdict URN: urn:ump:ermkvz6ifbp5svga5cnapv5jhm7c5b7qdbwjjerfpxbwcrolm2za (readback verified)
Codex quota-walled until 2026-08-29; GLM stand-in per Sami 2026-08-17 directive.

dashcaddy-api/src/utilities/config-schema.js

@@ -14,7 +14,11 @@ const KNOWN_KEYS = [
   'configurationType', 'defaults', 'customLogo', 'customFavicon',
   'dashboardTitle', 'tailscale', 'license', 'skipped',
   'routingMode', 'domain', 'email', 'defaultIP', 'pylon',
-  'customLogoDark', 'customLogoLight', 'language'
+  'customLogoDark', 'customLogoLight', 'language',
+  // license-manager.js persists the last activation to config.licenseBackup
+  // (restore-on-restart path); src/config/migrations.js stamps _version.
+  // Both are first-party writes — see DC-091.
+  'licenseBackup', '_version'
 ];
 
 /**