docs: Add branch and PR naming conventions

CONTRIBUTING.md

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+# Contributing to plex-playlist
+
+## Branch Naming Strategy
+
+Use the following pattern for branch names:
+
+```
+[type]/PP-[number]-brief-description
+```
+
+### Pattern Components
+
+- **type**: Feature category
+  - `feat/` – New feature
+  - `bug/` – Bug fix
+  - `docs/` – Documentation
+  - `refactor/` – Code restructuring
+  - `chore/` – Maintenance, dependencies, tooling
+  - `test/` – Test additions or fixes
+  - `perf/` – Performance improvements
+
+- **PP-[number]**: Issue tracker prefix + issue number
+  - Example: `PP-10`, `PP-15`, `PP-8`
+
+- **brief-description**: Kebab-case, lowercase, concise
+  - Example: `upgrade-backend-runtime`, `fix-startup-crash`
+
+### Examples
+
+```
+feat/PP-10-upgrade-backend-runtime
+bug/PP-15-fix-startup-crash
+docs/PP-8-add-api-endpoints
+refactor/PP-12-extract-compatibility-checks
+chore/PP-7-update-dependencies
+```
+
+### Useful Git Commands
+
+List all feature branches for this project:
+```sh
+git branch -l 'feat/PP-*'
+```
+
+List all branches for a specific issue:
+```sh
+git branch -l '*/PP-10-*'
+```
+
+## Pull Request Conventions
+
+### Title Format
+
+Keep PR titles human-friendly and descriptive:
+```
+Upgrade backend to Python 3.14 with exact dependency pinning
+```
+
+Or if you prefer structured titles:
+```
+[feat] PP-10: Upgrade backend to Python 3.14 with exact dependency pinning
+```
+
+### PR Description
+
+Include the issue reference so Gitea can auto-link:
+```markdown
+Fixes PP-10
+
+## Summary
+Brief description of changes.
+
+## Changes
+- Bullet 1
+- Bullet 2
+
+## Testing
+How to verify this PR works.
+```
+
+## Commit Message Style
+
+Commits use standard Git convention with optional type prefix:
+
+```
+[type] Brief description (50 chars max)
+
+Optional longer body with more context.
+Wrap at 72 characters.
+
+Fixes PP-10
+```
+
+Example:
+```
+feat: Add startup compatibility validation
+
+Implement runtime policy checks to fail fast if Python version
+or pinned package versions don't match deployment baseline.
+
+Fixes PP-10
+```
+
+## Code Quality Gates
+
+All commits must pass local pre-commit hooks:
+- `ruff` (format + lint)
+- `pyright` (type checking)
+- `pytest` (tests + coverage ≥95%)
+- `pydoclint` (docstring validation)
+- `xdoctest` (doctest extraction)
+
+Run locally before pushing:
+```sh
+cd backend
+uv run pytest
+```
+
+## Development Environment
+
+See [README.md](README.md#manual-setup-if-not-using-docker) for manual setup.
+
+For Docker development:
+```sh
+docker compose -f compose.dev.yml up
+```
+
+## Questions or Issues?
+
+Refer to project documentation in `docs/` or open an issue on Gitea.