Run the database version the rest of the product already recommends

INSTALL-WINDOWS-IIS.md has said MySQL 8.4 LTS is standard for new installs since
8.0 reached end of life in April 2026, while both compose files and the offline
bundler still pinned 8.0. A site reading the Windows runbook and a site reading
the Docker one were being told to run different servers, and the migration page
written this week sent people onto the dead one.

Verified against a real server rather than by editing a tag: 8.4.11, core chain
plus five plugin chains applied clean, 66 tables at a single utf8mb4_unicode_ci
collation, six alembic version tables. The image's PyMySQL authenticates against
8.4's caching_sha2_password, which is what requirements.in already pins
cryptography for.

Existing servers need one thing done FIRST: 8.4 removes mysql_native_password,
so an account created on 5.6 or 5.7 must be moved to caching_sha2_password
before the upgrade or it cannot authenticate afterwards. In-place also has no
downgrade path, and 5.7 cannot reach 8.4 in one hop. For databases this size a
dump into a fresh 8.4 server is the better trade: same outage, and the old
server stays as the rollback.

Air-gapped sites need a fresh offline bundle, because the tarball carries the
MySQL image alongside the app image.

Also here, found by having it bite during that verification: the db healthcheck
pinged over the unix socket, and the entrypoint's init pass answers on the
socket while running the server with --skip-networking. The probe therefore
reported healthy DURING init, which is what `depends_on: service_healthy` gates
api and migrate on. A ping passed at 8 seconds and the next query failed because
the server was mid-restart. Probing 127.0.0.1 keeps it red until the real server
is listening.

docs/DEPLOY-AIRGAP.md

@@ -27,7 +27,7 @@ pwsh scripts/build-offline-bundle.ps1 -Version 0.7.0
 ```
 
 This builds `shopdb-flask:0.7.0` (frontend + all Python deps baked in), pulls
-`mysql:8.0`, and writes:
+`mysql:8.4`, and writes:
 
 - `shopdb-stack-0.7.0.tar.gz` - both images in one archive
 - `shopdb-stack-0.7.0.tar.gz.sha256` - checksum to verify after transfer