chore: keep issue screenshots out of the source tree (#1260)

Preventative twin of the main-side cleanup. This branch has no stray images
to remove — it just gets the same guard, so the two branches agree and a
backport cannot carry one across.

Two PR screenshots landed at main's repo root in #1241 and shipped as part of
the source tree. Screenshots belong on a `screenshots/*` branch, which is how
every other UI change here has attached its evidence.

Anchored with a leading slash so docs/ keeps its own images and test-assets/
keeps the fixtures the e2e specs load. Verified no tracked file on this branch
matches the new patterns.

Co-authored-by: Paul Nothaft <paul@MacStudio-von-Paul.local>

.gitignore

@@ -141,3 +141,16 @@ backend/storage/
 # committed 16 .pyc files here in #1247.
 __pycache__/
 *.pyc
+
+# Issue / PR screenshots belong on a `screenshots/*` branch, never on main or
+# stable — that is what those branches exist for. Two landed at the repo root
+# on main in #1241 and shipped as part of the source tree; nothing stopped it.
+#
+# Anchored with a leading slash so docs/ keeps its own images.
+/issue-*.png
+/issue-*.jpg
+/screenshot-*.png
+/screenshot-*.jpg
+/*-screenshot.png
+/*-screenshot.jpg
+