docs: add API reference and enrich OpenAPI metadata

- docs/API.md: full REST + WebSocket reference with auth usage
  (cookie/Bearer login flow, curl examples), conventions, endpoint
  tables, and WS protocol (WS routes are not in the OpenAPI schema)
- server.py: app-level description, openapi_tags grouping, and
  tags/responses metadata on all REST routes documenting the error
  codes each route raises; hide the SPA catch-all from the schema
- README: link the API reference in the docs index

README.md

@@ -205,6 +205,7 @@ The GPU key can be a UUID, `pci:XXXX`, or `idx:N` fallback. Find your GPU key wi
 - **[Overview](docs/Overview.md)** — What it does, capabilities, architecture
 - **[Installation](docs/Installation.md)** — Prerequisites, source build, troubleshooting
 - **[Usage Guide](docs/Usage-Guide.md)** — Web UI, CLI reference, systemd service
+- **[API Reference](docs/API.md)** — REST + WebSocket API, authentication, endpoint summary (interactive docs at `/docs`)
 - **[Tips and Tricks](docs/Tips-and-Tricks.md)** — Workflows, curve flattening, safety
 - **[WireView Pro II Setup](docs/WireView.md)** — One-time host setup (udev rule + port access) for the 12VHPWR connector monitor