feat: M4 in-admin manual generation on chained-cron checkpoints + IA/perf pass (v0.21.0) (#166)

Final Phase 2 admin milestone. Board "New Article" runs through the M3 chained-cron checkpoint state machine (one stage/idea per tick, queue persisted before generate, lock released on every exit path) so generation survives Hostinger background-kill; retires the SSH-only do_action workaround (WP-CLI routes through the same entry). IA/perf: settings-backed board/ideas pagination + db 1.4.0 indexes. Folds guard P2s (--self-test + window doc).

QA: APPROVE-WITH-P2-NOTES @ f2a00d4 + delta APPROVE @ 0c8071f. Deferred (tracked): class-admin-page.php 353-line split + pagination Settings-UI fields.

Agent-Session: phase2-admin-m4-20260614

.github/workflows/ci.yml

@@ -126,3 +126,7 @@ jobs:
         # by defined() at the reference site. Prevents the v0.19.0 incident
         # class (constant deleted from bootstrap, test fallback masks the gap).
         run: php scripts/check-constants.php
+      - name: Self-test check-constants guard (v0.21.0)
+        # Verifies the guard script itself catches a synthetic missing constant.
+        # Exit 2 = self-test failure (scanner is broken). Exit 0 = scanner works.
+        run: php scripts/check-constants.php --self-test