Add env examples and improve setup docs

[hsusul]

Summary

• Add backend and frontend .env.example files with safe placeholder values
• Replace default/incomplete setup docs with app-specific backend and frontend instructions
• Update the root README with concise local development steps and links to setup docs

Validation

• git diff --check

Notes

• Did not run backend checks because Django is not installed in the active Python environment.
• Did not run the frontend build because local node_modules is missing react-ga4.

[vercel]

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