Add pytest suite and Gitea Actions CI/CD pipeline

83 tests (94% coverage of src/py) covering the builder pipeline
(gallery sync, HTML/CSS generation, image processing, full site
build) and the Flask webui (routes, uploads, theme/font management).

Dockerfile gains a `test` stage (pytest) between the wheel builder
and the prod image, and pins the alpine base to a full patch tag so
Renovate can classify updates. CI workflow builds, smoke-tests, runs
the suite, scans with Trivy, and publishes/releases on merge to main,
following the same pipeline already running on adguard-cidre.

.gitignore

@@ -2,5 +2,9 @@
 !.env
 !.sh
 !.gitignore
+!.gitea
+!.gitea/**
 output/
-__pycache__/
+__pycache__/
+htmlcov/
+site_output.zip