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/main.py

@@ -16,7 +16,7 @@ from datetime import timezone
 from decimal import Decimal
 
 import httpx
-from openai import AsyncOpenAI
+from anthropic import AsyncAnthropic
 from sqlalchemy import select
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 
@@ -179,14 +179,9 @@ async def run() -> None:
     # Database
     engine, session_factory = create_db(config)
 
-    # OpenRouter client + LLM analyzer
-    client = AsyncOpenAI(
-        api_key=config.openrouter_api_key,
-        base_url="https://openrouter.ai/api/v1",
-        default_headers={
-            "HTTP-Referer": "https://trading.viktorbarzin.me",
-            "X-Title": "trading-bot meet-kevin",
-        },
+    # Anthropic client + LLM analyzer (OAuth bearer token)
+    client = AsyncAnthropic(
+        auth_token=config.anthropic_oauth_token,
     )
     analyzer = LlmAnalyzer(
         client=client,