Add CI with checks and automated release builds

Gitea Actions workflow (.gitea/workflows/ci.yml):

- check job: ruff format --check, ruff check, ty check on pushes to main
- release job on v* tags: build a standalone Linux executable with
  PyInstaller (--onefile), smoke-test it, create a Gitea release via
  the API and upload the binary as an asset

Also add a pyinstaller build dependency group and bump to 0.3.0.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "weekly-activity"
-version = "0.2.0"
+version = "0.3.0"
 description = "Weekly activity report (merge proposals, pull requests, bugs, issues) for Launchpad and GitHub."
 requires-python = ">=3.11"
 dependencies = [
@@ -19,6 +19,7 @@ build-backend = "hatchling.build"
 
 [dependency-groups]
 dev = ["ruff>=0.6", "ty>=0.0.1"]
+build = ["pyinstaller>=6"]
 
 [tool.hatch.build.targets.wheel]
 packages = ["src/weekly_activity"]