feat(ai): self-host settings UI for Anthropic provider (5/5)

Adds the Anthropic panel and the install-wide LLM provider selector to
the self-hosting settings page, plus a shared data-retention
disclosure that covers both OpenAI and Anthropic.

- New _llm_provider_selector partial: select for Setting.llm_provider
  (openai | anthropic), respects the LLM_PROVIDER env var (disables the
  control + shows the "configured through environment variables" hint
  when set, mirroring the existing OpenAI panel behaviour), and renders
  a compact data-handling block with one-line retention statements for
  each provider.
- New _anthropic_settings partial mirrors _openai_settings exactly:
  password-field for the API key with **** redaction, optional
  base_url (for AWS Bedrock / GCP Vertex), optional default model. All
  three fields disable when their ENV var is set.
- show.html.erb renders provider selector + OpenAI panel + Anthropic
  panel under the same "General" section so users can configure either
  (or both) without switching pages.
- Settings::HostingsController#update now permits and persists
  anthropic_access_token (ignoring the **** placeholder, same pattern
  as OpenAI), anthropic_base_url, anthropic_model, and llm_provider
  (validated against %w[openai anthropic]). On Setting::ValidationError
  the rescue branch preserves anthropic_base_url / anthropic_model
  input so the form re-renders with the user's typed values intact —
  parity with the issue #1824 fix for OpenAI.
- Locale keys added under settings.hostings.{llm_provider_selector,
  anthropic_settings}.

Tests cover token update + placeholder redaction, base_url + model
update, llm_provider switch to anthropic, and rejection of unknown
provider values. The existing GET render test still passes, exercising
all three new partials.

Closes the 5/5 Anthropic series stacked on #1986.

test/controllers/settings/hostings_controller_test.rb

@@ -74,6 +74,51 @@ class Settings::HostingsControllerTest < ActionDispatch::IntegrationTest
     end
   end
 
+  test "can update anthropic access token when self hosting is enabled" do
+    with_self_hosting do
+      patch settings_hosting_url, params: { setting: { anthropic_access_token: "sk-ant-test" } }
+
+      assert_equal "sk-ant-test", Setting.anthropic_access_token
+    end
+  end
+
+  test "ignores redacted anthropic token placeholder" do
+    with_self_hosting do
+      Setting.anthropic_access_token = "previous-token"
+
+      patch settings_hosting_url, params: { setting: { anthropic_access_token: "********" } }
+
+      assert_equal "previous-token", Setting.anthropic_access_token
+    end
+  end
+
+  test "can update anthropic base_url and model" do
+    with_self_hosting do
+      patch settings_hosting_url, params: { setting: { anthropic_base_url: "https://bedrock.example.com", anthropic_model: "claude-opus-4-7" } }
+
+      assert_equal "https://bedrock.example.com", Setting.anthropic_base_url
+      assert_equal "claude-opus-4-7", Setting.anthropic_model
+    end
+  end
+
+  test "can update llm_provider to anthropic" do
+    with_self_hosting do
+      patch settings_hosting_url, params: { setting: { llm_provider: "anthropic" } }
+
+      assert_equal "anthropic", Setting.llm_provider
+    end
+  end
+
+  test "rejects unknown llm_provider values" do
+    with_self_hosting do
+      Setting.llm_provider = "openai"
+
+      patch settings_hosting_url, params: { setting: { llm_provider: "bogus" } }
+
+      assert_equal "openai", Setting.llm_provider
+    end
+  end
+
   test "can update openai uri base and model together when self hosting is enabled" do
     with_self_hosting do
       patch settings_hosting_url, params: { setting: { openai_uri_base: "https://api.example.com/v1", openai_model: "gpt-4" } }