ci: publish images to the Gitea registry, use the gitea context

Release now pushes to git.lucasrguerra.dev.br instead of ghcr.io, tags each
architecture separately (no artifact upload, no gha cache) and joins them with
imagetools. GITHUB_OUTPUT and GITHUB_STEP_SUMMARY stay: they are the runner's
own interface and the GITEA_* equivalents are not set.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.env.example

@@ -1,10 +1,10 @@
 # --- Container images ---
-# Published to GHCR by .github/workflows/release.yml, multi-architecture
+# Published to the Gitea registry by .gitea/workflows/release.yml, multi-architecture
 # (linux/amd64 and linux/arm64). The deploy host pulls these instead of
 # compiling: `docker compose pull && docker compose up -d`. Running
 # `docker compose build` locally still overrides them with a local build.
 # Pin IMAGE_TAG to a released version (e.g. 1.2.3) or a sha- tag to roll back.
-IMAGE_NAMESPACE=ghcr.io/lucasrguerra/lambdamail
+IMAGE_NAMESPACE=git.lucasrguerra.dev.br/lucasrguerra/lambdamail
 IMAGE_TAG=latest
 
 # --- Postgres ---