ci: add Gitea Actions workflow for Docker build and registry push

Adds a CI pipeline that builds the Docker image and pushes it to the
Gitea container registry on every push to master. Includes .dockerignore
to keep the build context lean.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,15 @@
+.git
+.gitignore
+.env
+.env.example
+data/
+docs/
+tests/
+alembic/
+*.md
+*.lock
+.claude/
+.gitea/
+__pycache__
+*.pyc
+docker-compose.yaml