refactor: comprehensive code quality, security, and release readiness improvements

Security: tighten CORS defaults, add webhook rate limiting, fix XSS in
automations, guard WebSocket JSON.parse, validate ADB address input,
seal debug exception leak, URL-encode WS tokens, CSS.escape in selectors.

Code quality: add Pydantic models for brightness/power endpoints, fix
thread safety and name uniqueness in DeviceStore, immutable update
pattern, split 6 oversized files into 16 focused modules, enable
TypeScript strictNullChecks (741→102 errors), type state variables,
add dom-utils helper, migrate 3 modules from inline onclick to event
delegation, ProcessorDependencies dataclass.

Performance: async store saves, health endpoint log level, command
palette debounce, optimized entity-events comparison, fix service
worker precache list.

Testing: expand from 45 to 293 passing tests — add store tests (141),
route tests (25), core logic tests (42), E2E flow tests (33), organize
into tests/api/, tests/storage/, tests/core/, tests/e2e/.

DevOps: CI test pipeline, pre-commit config, Dockerfile multi-stage
build with non-root user and health check, docker-compose improvements,
version bump to 0.2.0.

Docs: rewrite CLAUDE.md (202→56 lines), server/CLAUDE.md (212→76),
create contexts/server-operations.md, fix .js→.ts references, fix env
var prefix in README, rewrite INSTALLATION.md, add CONTRIBUTING.md and
.env.example.

CONTRIBUTING.md

@@ -0,0 +1,85 @@
+# Contributing
+
+## Prerequisites
+
+- Python 3.11+
+- Node.js 20+ (for frontend bundle)
+- Git
+
+## Development Setup
+
+```bash
+git clone https://git.dolgolyov-family.by/alexei.dolgolyov/wled-screen-controller-mixed.git
+cd wled-screen-controller-mixed/server
+
+# Python environment
+python -m venv venv
+source venv/bin/activate   # Linux/Mac
+# venv\Scripts\activate    # Windows
+pip install -e ".[dev]"
+
+# Frontend dependencies
+npm install
+npm run build
+```
+
+## Running the Server
+
+```bash
+cd server
+export PYTHONPATH=$(pwd)/src   # Linux/Mac
+# set PYTHONPATH=%CD%\src      # Windows
+python -m wled_controller.main
+```
+
+Open http://localhost:8080 to access the dashboard.
+
+## Running Tests
+
+```bash
+cd server
+pytest
+```
+
+Tests use pytest with pytest-asyncio. Coverage reports are generated automatically.
+
+## Code Style
+
+This project uses **black** for formatting and **ruff** for linting (both configured in `pyproject.toml` with a line length of 100).
+
+```bash
+cd server
+black src/ tests/
+ruff check src/ tests/
+```
+
+## Frontend Changes
+
+After modifying any file under `server/src/wled_controller/static/js/` or `static/css/`, rebuild the bundle:
+
+```bash
+cd server
+npm run build
+```
+
+The browser loads the esbuild bundle (`static/dist/`), not the source files directly.
+
+## Commit Messages
+
+Follow the [Conventional Commits](https://www.conventionalcommits.org/) format:
+
+```
+feat: add new capture engine
+fix: correct LED color mapping
+refactor: extract filter pipeline
+docs: update API reference
+test: add audio source tests
+chore: update dependencies
+```
+
+## Pull Requests
+
+1. Create a feature branch from `master`
+2. Make your changes with tests
+3. Ensure `ruff check` and `pytest` pass
+4. Open a PR with a clear description of the change