Add 837P parser design spec

- Approved design for the Python 837P parser module under backend/
- Pydantic v2, click CLI, structural + CO Medicaid validation
- One JSON file per claim, summary.json, continue-on-failure
- Includes .gitignore for the Python + Node stack

.gitignore

@@ -0,0 +1,33 @@
+# Dependencies
+node_modules/
+.venv/
+venv/
+__pycache__/
+*.py[cod]
+*.egg-info/
+.pytest_cache/
+.ruff_cache/
+
+# Build output
+dist/
+build/
+*.tsbuildinfo
+
+# Editor / OS
+.DS_Store
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# Local config
+.env
+.env.local
+.env.*.local
+
+# Production data (handled by ops, not committed)
+docs/prodfiles/*/
+*.production.txt
+
+# Local parser output
+claims_output/