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

services/meet_kevin_watcher/config.py

@@ -18,12 +18,12 @@ class MeetKevinWatcherConfig(BaseConfig):
 
     # LLM analysis settings
     meet_kevin_max_llm_retries: int = 3
-    meet_kevin_llm_model: str = "anthropic/claude-sonnet-4.5"
+    meet_kevin_llm_model: str = "claude-sonnet-4-5"
     meet_kevin_prompt_version: str = "v1"
     meet_kevin_daily_cost_cap_usd: float = 5.0
 
     # API credentials
-    openrouter_api_key: str = ""
+    anthropic_oauth_token: str = ""
 
     # Runtime settings
     meet_kevin_workdir: str = "/tmp/meet_kevin_captions"