chore(rmi-backend,audit): gitignore main.py.bak

Phase 1 of AUDIT-2026-Q3.md item P1.4.

main.py.bak was a 374KB legacy leftover from the old 220-file app/
monolith. Its removal from the working tree was completed in e404e90
(feat(rmi-backend,audit): add app/main.py entrypoint delegating to
factory) in this audit session — that commit deleted main.py.bak and
added app/main.py in a single change.

This commit adds the defensive .gitignore entry so any future
main.py.bak / similar leftover files do not get re-tracked.

main.py.bak archive kept at:
  /tmp/rmi-backend-archive-2026-07/main.py.bak.removed-2026-07-06

Note: historical commit bde2f3a still contains 4 mainnet token
addresses for gitleaks; full rewrite with git-filter-repo is scheduled
for Phase 1.11 of AUDIT-2026-Q3.md.

.gitignore

@@ -87,3 +87,4 @@ data/papers/
 *.gguf
 *.h5
 *.hdf5
+main.py.bak