feat: Anthropic SDK backend — API key alternative to Claude CLI OAuth

Adds `anthropic_api` model type so users can authenticate with a direct
Anthropic API key instead of (or alongside) the CLI OAuth session.

- model_registry.py: `anthropic_api` type; `save/get/remove_anthropic_api_key()`
  mirroring the Google account pattern; `save_cloud_model()` now picks type
  based on credential type (cli → claude_cli, api_key → anthropic_api);
  `_resolve_model()` merges api_key from the credential entry
- llm_client.py: `_anthropic_api()` backend (AsyncAnthropic SDK); dispatch
  and fallback wiring; usage tracking
- routers/local_llm.py: Anthropic API key management routes
  (POST /settings/local/anthropic-key, /anthropic-key/{id}/remove);
  `anthropic_api` badge and edit-form credential selector
- static/local_llm.html: Anthropic Cloud Provider block now shows API key
  management (add/remove); Add Model → Anthropic tab has credential selector
  (CLI vs API key)
- requirements.txt: enable anthropic>=0.40.0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/requirements.txt

@@ -31,5 +31,5 @@ pywebpush>=2.0.0
 # MariaDB / MySQL connector — used by ae_db_query orchestrator tool
 pymysql>=1.1.0
 
-# anthropic SDK not needed — using claude CLI subprocess for auth
-# anthropic>=0.40.0
+# Anthropic SDK — direct API key backend (alternative to CLI OAuth)
+anthropic>=0.40.0