feat: add Brandt Smart Control authentication

[iMicknl]

Summary

• Add a dedicated BrandtAuthStrategy implementing Brandt's two-layer auth: log in to the Smart Control middleware (smartcontrol-app.com, a cookie-session Rails API), fetch a JWT using the resulting session cookie, then log in to the Overkiz cloud with jwt=<JWT> only.
• The two middleware calls share the client's aiohttp.ClientSession cookie jar, so the JWT GET is authenticated purely by the session cookie set on login.
• Add BRANDT_MIDDLEWARE_API / BRANDT_PARTNER constants and BrandtBadCredentialsError / BrandtServiceError exceptions, following the Cozytouch/Nexity precedent.
• Route Server.BRANDT to the new strategy in the auth factory (it previously fell through to the generic SessionLoginStrategy and silently failed).
• Update README/docs: Brandt is now supported, removing the obsolete "authentication not supported" footnote.

Test Plan

• pytest tests/test_auth.py — new TestBrandtAuthStrategy (success flow, bad credentials, missing JWT) + factory routing test, all passing
• Full suite: 546 passed
• ruff + ty clean
• Optional: live verification against a real Brandt account (the Smart Control app is discontinued; the service may no longer be operational)