Remove the checked-in `pybind_kv.so` binary and add a pre-commit hook to prevent regressions

[devangb3]

backend/pybind_kv.so is a 9.5 MB compiled artifact checked into Git. It's not used (the KV service talks to ResilientDB over HTTP).

Current state:
backend/pybind_kv.so present, not imported.

Proposed implementation:

1. Delete the file. git filter-repo --path backend/pybind_kv.so --invert-paths to rewrite history (communicate this — it's force-push territory; only do it if you own the upstream).
2. Add *.so to .gitignore (already present for some, verify).
3. Add a pre-commit hook (pre-commit framework) that rejects files over 1 MB unless allowlisted.
4. Document in docs/contributing.md.

Files likely affected:

• backend/pybind_kv.so (deleted)
• .gitignore
• .pre-commit-config.yaml (new)

Acceptance criteria:

• git log -- backend/pybind_kv.so no longer shows the file.
• A commit that adds a 2 MB binary is rejected locally by pre-commit.