refactor(meet-kevin): switch LLM back to native Anthropic SDK with OAuth bearer

Previous refactor (89f01ad) moved to OpenRouter because no sk-ant-api-* key
was found in Vault. Turns out claude-agent-service-spare-{1,2} hold
sk-ant-oat01-* OAuth tokens (108 chars, scope user:inference, 1-year TTL,
minted via 'claude setup-token' — see memory id=832).

These tokens work with the Anthropic SDK via the auth_token= constructor
argument (routes to Authorization: Bearer ... instead of x-api-key: ...).
They consume the Enterprise Claude subscription quota rather than
per-call billing, so the OpenRouter zero-credit problem goes away.

- llm_analyzer.py: revert OpenAI client to AsyncAnthropic; tool-use API
  + cache_control restored
- config.py: openrouter_api_key -> anthropic_oauth_token; model slug
  reverted from anthropic/claude-sonnet-4.5 -> claude-sonnet-4-5
- main.py: AsyncOpenAI -> AsyncAnthropic(auth_token=...), drop OpenRouter
  attribution headers
- pyproject: openai>=1.50 -> anthropic>=0.40 in meet_kevin extras
- tests: mocks ported back to messages.create + tool_use blocks

pyproject.toml

@@ -20,7 +20,7 @@ news = ["feedparser>=6.0", "praw>=7.7", "asyncpraw>=7.7", "httpx>=0.27"]
 sentiment = ["transformers>=4.38", "torch>=2.2", "ollama>=0.1"]
 trading = ["alpaca-py>=0.21", "pytz>=2024.1", "yfinance>=0.2", "httpx>=0.27"]
 backtester = ["numpy>=1.26", "pandas>=2.2"]
-meet_kevin = ["yt-dlp>=2025.12", "feedparser>=6.0", "openai>=1.50", "httpx>=0.27"]
+meet_kevin = ["yt-dlp>=2025.12", "feedparser>=6.0", "anthropic>=0.40", "httpx>=0.27"]
 dev = ["pytest>=8.0", "pytest-asyncio>=0.23", "pytest-cov>=4.1", "ruff>=0.3", "mypy>=1.8", "httpx>=0.27"]
 
 [build-system]