chore(lint): set ruff line-length 100 and gate ruff format --check

Adds ruff's formatter to the gate. line-length 100 matches the frontend
prettier printWidth. The reformat itself lands as the next commit (kept
separate so it can go in .git-blame-ignore-revs).

backend/ruff.toml

@@ -2,5 +2,11 @@
 # high-signal pyflakes/pycodestyle checks that catch real dead code + bugs), but ignores E402:
 # a handful of modules (e.g. main.py) deliberately run setup — logging config, sys.path — before
 # importing route modules, so "module import not at top of file" is intentional there, not cruft.
+#
+# line-length 100 (ruff's default is 88): matches the frontend prettier printWidth so both halves of
+# the repo wrap at the same width, and it's the width `ruff format` enforces (gated via
+# `ruff format --check` alongside `ruff check`).
+line-length = 100
+
 [lint]
 ignore = ["E402"]