Stage A on variants that need a database at startup #6

Found while implementing spec 001 tooling (PR #5). Stage A only verifies json and plaintext, but many variants that declare a database open the connection at startup and exit when it is missing. For example, Go/gin:default fails with lookup tfb-database ... no such host.

Numbers from catalog.py check on main:

• 965 variants, 160 of them broken;
• 203 variants have no database;
• the rest use postgres (507), mysql (209) or mongodb (46).

Decide:

• Weekly full run: verify stage A only on variants with database: none, or classify DB startup failures as needs-database instead of fail.
• Or bring the spec 002 database (dataset-* through dataset-tool#1) forward and start it on the private network during stage A.
• Update the plan §4 smoke-sample example. It lists Go/gin and Java/spring; the CI now uses DB-free variants: C/libreactor, Go/go-std, Java/javalin, JavaScript/express, Python/fastpysgi, Rust/axum.
• Track the 39 existing Dockerfile tag problems (14 :latest, 25 untagged). CI only lints Dockerfiles changed in a PR.