ci: lint workflows and shell scripts, validate k8s against the API server

Adds three lint jobs (actionlint, shellcheck, compose) and a server-side
dry-run of the active manifests. Previously the only k8s check was
kubeconform, which has no schemas for CRDs, so every IngressRoute,
Certificate, PrometheusRule and Middleware was silently skipped.

The server-side pass needs the live API server because that is the only
place the real CRD schemas and the cert-manager / Traefik admission
webhooks exist. It is scoped to services carrying a k8s/active marker,
since dry-run needs the target namespace to exist. userbot/ is excluded
from shellcheck: it is a git subtree, and linting upstream's scripts would
let a routine subtree pull turn the deploy gate red on code we do not own.

kubeconform, shellcheck and actionlint are now installed from pinned
versions in tool-versions.env rather than picked up from the runner's
PATH. The Compose helper is shared with the deploy workflow so both
check the same file set the same way.

.gitea/workflows/compose-lint.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# Shared helpers for validating Compose files. Sourced both by steps in
+# .gitea/workflows/ci.yaml and by deploy-lib.sh on the workstation.
+#
+# Two levels of checking, matching how the repo is structured:
+#
+#   general  every committed Compose file, active or not. Pure structure check:
+#            no ${VAR} interpolation, no .env lookup, no bind-mount path
+#            resolution. Disabled stacks deliberately have no .env in the repo
+#            and no values on the CI runner, so a full `config` run would fail on
+#            their `${VAR:?}` guards for reasons that have nothing to do with the
+#            change under review.
+#
+#   full     active stacks only, with interpolation and env-file resolution, so
+#            required variables and referenced files are actually resolved. Needs
+#            the gitignored .env files, so this only runs in the deploy workflow
+#            on the workstation.
+#
+# This file is meant to be sourced, not executed.
+
+# All committed Compose files, including the ones deploy never starts.
+compose_files() {
+  git ls-files \
+    '*/compose.yaml' '*/compose.yml' 'compose.yaml' 'compose.yml' \
+    '*/docker-compose.yaml' '*/docker-compose.yml'
+}
+
+# Prints the flags that turn `docker compose config` into the general check.
+# Probed rather than hardcoded so an older Compose without --no-env-resolution
+# still gets the flags it does support.
+compose_safe_flags() {
+  local help flag
+  help="$(docker compose config --help 2>/dev/null || true)"
+  for flag in --no-interpolate --no-env-resolution --no-path-resolution; do
+    if printf '%s' "$help" | grep -q -- "$flag"; then
+      printf '%s\n' "$flag"
+    fi
+  done
+}
+
+# validate_compose_file <file> [extra docker compose config flags...]
+validate_compose_file() {
+  local file="$1"
+  shift
+  docker compose -f "$file" config --quiet "$@"
+}