Drop the committed __pycache__ and add a .gitignore

app/__pycache__/app.cpython-313.pyc was committed and went stale the moment
app.py changed. It was inert because the image runs 3.12 and the bytecode was
3.13, but a stale .pyc that happened to match the interpreter would be a very
quiet way to run the wrong code.

Also ignores service_account.json, which is bind-mounted in at runtime and has
no business being committable.

.gitignore

@@ -0,0 +1,3 @@
+__pycache__/
+*.py[cod]
+service_account.json