Backend runtime upgraded to Python 3.14 with exact dependency pinning (#57)

Signed-off-by: Cliff Hill <xlorep@darkhelm.org>

## Summary

Upgrades backend runtime baseline and dependency management for issue #10.

### Changes

1. **Python Baseline**: Updated from 3.13 to 3.14
   - Updated `backend/pyproject.toml` requires-python constraint
   - Updated `backend/pyrightconfig.json` pythonVersion
   - Updated all Dockerfile and CI references

2. **Dependency Pinning**: Switched to exact version pins in `backend/pyproject.toml`
   - All dev and runtime dependencies now use `==` instead of `>=`
   - `fastapi==0.120.2`, `uvicorn==0.38.0`
   - ruff, pyright, pytest suite pinned to current resolved versions
   - Regenerated `backend/uv.lock` under Python 3.14

3. **Startup Compatibility Guard** (TDD via RED→GREEN)
   - New `compatibility_status()` function evaluates runtime and pinned deps
   - Startup raises `RuntimeError` if policy fails
   - Implemented via FastAPI lifespan (non-deprecated) handler

4. **Compatibility Status Endpoint**
   - New `GET /compatibility` returns policy status, runtime version, and package checks
   - Shares single source of truth with startup validation

5. **Integration Tests**
   - Added failing-then-passing tests for startup guard and endpoint behavior
   - 100% coverage maintained

6. **Direnv Configuration**
   - Added `UV_PYTHON="3.14"` pin to repo `.envrc`
   - Ensures direnv creates/recreates venv with correct Python version

### Validation

- ✅ ruff format/check
- ✅ pyright strict (0 errors)
- ✅ pytest: 8 passed, 100% coverage (>=95 gate)
- ✅ pydoclint: pass
- ✅ xdoctest: pass

### Notes

- SQLAlchemy/SQLModel introduction deferred to next pass per scope
- Compatibility logic currently validates fastapi/uvicorn pins (runtime deps)
- Ready for container build validation and Renovate bot testing

Co-authored-by: copilotcoder <copilotcoder@darkhelm.org>
Reviewed-on: #57
Co-authored-by: Cliff Hill <xlorep@darkhelm.org>
Co-committed-by: Cliff Hill <xlorep@darkhelm.org>

CONTRIBUTING.md

@@ -0,0 +1,130 @@
+# 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.