feat: add Makefile for contributor workflow

[sundaram2021]

Description

Adds a root Makefile as the canonical entry point for common contributor commands (install, test, lint, format, build, serve, Docker) and updates all
contributor-facing docs to reference it. This gives contributors one consistent interface regardless of platform or shell.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

fixes #294

Changes Made

• Add root Makefile with targets: help, install, install-dev, test, test-all, lint, format, format-check, check, build, clean,
  serve, docker-build, docker-up, docker-up-sqlite, docker-up-postgres, docker-down, docker-logs
• Support interpreter overrides via PYTHON=/path/to/python and test overrides via TEST_ARGS="..."
• Update CONTRIBUTING.md to reference make install, make serve, make check, make test, make lint, make format
• Rewrite docs/DEVELOPER_SETUP.md around make targets; clean up mojibake from prior encoding
• Add make help hint to the Contributing section of README.md
• Log the change as Round 9 in CONTRIBUTIONS.md

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (make test)
• Lint passes (make lint)
• Manual testing performed — ran make help, make install, make check, make serve, and verified Docker targets invoke the correct compose
  profiles

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (N/A — Makefile/docs change, no runtime code added)
• New and existing unit tests pass locally with my changes

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