docs: write docs/architecture.md, plugin-development.md, getting-started.md, api.md

[strausmann]

Goal

Write user + contributor documentation referenced from README and CONTRIBUTING.

Tasks

• docs/architecture.md — diagram, components, data flow, status sources, queue lifecycle
• docs/plugin-development.md — step-by-step: copy pt_series.py as template, implement Protocol, register, test
• docs/getting-started.md — for end users, deployment + smartphone PWA install + notifications enable
• docs/api.md — auto-generated from FastAPI OpenAPI (consider redoc-cli or similar)
• All docs follow the trademark + privacy rules in CLAUDE.md