Add MIT license, contribution guide, and public repo docs

CONTRIBUTING.md

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+# Contributing
+
+Thanks for considering a contribution.
+
+## Setup
+
+```bash
+python -m venv .venv
+source .venv/bin/activate
+pip install -e '.[dev]'
+```
+
+Copy env template:
+
+```bash
+cp .env.example .env
+```
+
+Then set real values in `.env` and run:
+
+```bash
+streamlit run app.py
+```
+
+## Development guidelines
+
+- Keep architecture boundaries clear:
+  - `clients/` for external integrations
+  - `domain/` for normalization/business logic
+  - `services/` for app services/indexing
+  - `ui/` for Streamlit rendering
+- Avoid introducing optional fallback paths unless required.
+- Prefer small, focused functions and explicit session-state keys.
+- Preserve safe SSH behavior and path quoting.
+
+## Validation
+
+Before opening a merge request, run:
+
+```bash
+PYTHONPATH=src python -m py_compile app.py src/media_library_viewer/*.py src/media_library_viewer/clients/*.py src/media_library_viewer/domain/*.py src/media_library_viewer/services/*.py src/media_library_viewer/ui/*.py
+```
+
+## Security / secrets
+
+Never commit:
+
+- `.env`
+- `.streamlit/secrets.toml`
+- private keys or API tokens
+
+Use `.env.example` for documented placeholders only.
+
+## Pull requests
+
+Please include:
+
+- what changed
+- why it changed
+- how it was tested
+
+If behavior/requirements changed, also update `docs/REQUIREMENTS.md`.