Add Docker packaging and Gitea Actions CI/CD pipeline.

Containerize the app for local and production deploys, and publish images
to the Gitea container registry on main pushes and version tags.

Co-authored-by: Cursor <cursoragent@cursor.com>

.dockerignore

@@ -0,0 +1,25 @@
+.git
+.gitignore
+.claude
+.venv
+venv
+__pycache__
+*.pyc
+*.pyo
+*.log
+server.log
+
+# Secrets and local config (injected via compose env_file)
+backend/.env
+.env
+*.env
+
+# Runtime artifacts (mounted as volumes in compose)
+backend/uploads/
+backend/outputs/
+backend/.llm_cache/
+out/
+
+# Large local samples
+samples/
+*.pdf