docs: capture stale-GRAV_VERSION api-plugin-install failure (ce-compound)

Fresh prod install ran Grav rc.10 (stale .env.prod GRAV_VERSION), so GPM
would not serve the api plugin (needs >=2.0.4); admin2 (auth via /api/v1)
then 404'd login silently. Documents the dead ends (deploying api config,
gpm index -f, same-channel assumption) and the fix (self-upgrade core +
reinstall + bump .env.prod). Adds reciprocal 'same 2026-07-04 cutover'
cross-links across the three sibling deploy gotchas.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Np4cMQLF77i664CAQXySzU

docs/solutions/test-failures/new-user-grants-api-not-admin-on-admin2.md

@@ -78,5 +78,6 @@ This site runs **Admin2 only** (the classic `admin` plugin is disabled), so auto
 
 ## Related Issues
 - `docs/working/plans/2026-07-04-grav-2.0.4-upgrade.md` — the self-contained test-account infrastructure shipped alongside the Grav 2.0.4 upgrade.
+- Sibling gotchas from the same 2026-07-04 Grav 2.0.4 production cutover (all surface around admin2/api but with distinct root causes): `docs/solutions/integration-issues/stale-grav-version-blocks-api-plugin-install.md` (stale `GRAV_VERSION` → rc core → GPM won't serve the `api` plugin → login 404s) and `docs/solutions/integration-issues/grav-double-content-encoding-garbage-page.md` (double `Content-Encoding` header → garbage page).
 - `docs/solutions/architecture-patterns/dual-repo-submodule-workflow.md` — accounts live in the `user/` repo; the `testrunner` account is gitignored so it never reaches production.
 - GPX manager auth model (`access.admin.login: true` frontmatter + Login plugin) — see the project's GPX manager notes.