feat(gateway): add web search emulation for Anthropic API Key accounts

Inject web search capability for Claude Console (API Key) accounts that
don't natively support Anthropic's web_search tool. When a pure
web_search request is detected, the gateway calls Brave Search or Tavily
API directly and constructs an Anthropic-protocol-compliant SSE/JSON
response without forwarding to upstream.

Backend:
- New `pkg/websearch/` SDK: Brave and Tavily provider implementations
  with io.LimitReader, proxy support, and Redis-based quota tracking
  (Lua atomic INCR + TTL, DECR rollback on failure)
- Global config via `settings.web_search_emulation_config` (JSON) with
  in-process cache + singleflight, input validation, API key merge on
  save, and sanitized API responses
- Channel-level toggle via `channels.features_config` JSONB column
  (DB migration 101)
- Account-level toggle via `accounts.extra.web_search_emulation`
- Request interception in `Forward()` with SSE streaming response
  construction using json.Marshal (no manual string concatenation)
- Manager hot-reload: `RebuildWebSearchManager()` called on config save
  and startup via `SetWebSearchRedisClient()`
- 70 unit tests covering providers, manager, config validation,
  sanitization, tool detection, query extraction, and response building

Frontend:
- Settings → Gateway tab: Web Search Emulation config card with global
  toggle, provider list (add/remove, API key, priority, quota, proxy)
- Channels → Anthropic tab: web search emulation toggle with global
  state linkage (disabled when global off)
- Account Create/Edit modals: web search emulation toggle for API Key
  type with Toggle component
- Full i18n coverage (zh + en)

backend/internal/pkg/websearch/brave_test.go

@@ -0,0 +1,119 @@
+package websearch
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestBraveProvider_Name(t *testing.T) {
+	p := NewBraveProvider("key", nil)
+	require.Equal(t, "brave", p.Name())
+}
+
+func TestBraveProvider_Search_Success(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		require.Equal(t, "test-key", r.Header.Get("X-Subscription-Token"))
+		require.Equal(t, "application/json", r.Header.Get("Accept"))
+		require.Equal(t, "golang", r.URL.Query().Get("q"))
+		require.Equal(t, "3", r.URL.Query().Get("count"))
+
+		resp := braveResponse{}
+		resp.Web.Results = []braveResult{
+			{URL: "https://go.dev", Title: "Go", Description: "Go lang", Age: "1 day"},
+			{URL: "https://pkg.go.dev", Title: "Pkg", Description: "Packages"},
+			{URL: "https://tour.go.dev", Title: "Tour", Description: "A Tour of Go", Age: "3 days"},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer srv.Close()
+
+	p := NewBraveProvider("test-key", srv.Client())
+	// Override the endpoint for testing
+	origURL := *braveSearchURL
+	u, _ := http.NewRequest("GET", srv.URL, nil)
+	*braveSearchURL = *u.URL
+	defer func() { *braveSearchURL = origURL }()
+
+	resp, err := p.Search(context.Background(), SearchRequest{Query: "golang", MaxResults: 3})
+	require.NoError(t, err)
+	require.Len(t, resp.Results, 3)
+	require.Equal(t, "https://go.dev", resp.Results[0].URL)
+	require.Equal(t, "Go lang", resp.Results[0].Snippet)
+	require.Equal(t, "1 day", resp.Results[0].PageAge)
+}
+
+func TestBraveProvider_Search_DefaultMaxResults(t *testing.T) {
+	var receivedCount string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		receivedCount = r.URL.Query().Get("count")
+		resp := braveResponse{}
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer srv.Close()
+
+	p := NewBraveProvider("key", srv.Client())
+	origURL := *braveSearchURL
+	u, _ := http.NewRequest("GET", srv.URL, nil)
+	*braveSearchURL = *u.URL
+	defer func() { *braveSearchURL = origURL }()
+
+	_, _ = p.Search(context.Background(), SearchRequest{Query: "test", MaxResults: 0})
+	require.Equal(t, "5", receivedCount)
+}
+
+func TestBraveProvider_Search_HTTPError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(429)
+		w.Write([]byte("rate limited"))
+	}))
+	defer srv.Close()
+
+	p := NewBraveProvider("key", srv.Client())
+	origURL := *braveSearchURL
+	u, _ := http.NewRequest("GET", srv.URL, nil)
+	*braveSearchURL = *u.URL
+	defer func() { *braveSearchURL = origURL }()
+
+	_, err := p.Search(context.Background(), SearchRequest{Query: "test"})
+	require.ErrorContains(t, err, "brave: status 429")
+}
+
+func TestBraveProvider_Search_InvalidJSON(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Write([]byte("not json"))
+	}))
+	defer srv.Close()
+
+	p := NewBraveProvider("key", srv.Client())
+	origURL := *braveSearchURL
+	u, _ := http.NewRequest("GET", srv.URL, nil)
+	*braveSearchURL = *u.URL
+	defer func() { *braveSearchURL = origURL }()
+
+	_, err := p.Search(context.Background(), SearchRequest{Query: "test"})
+	require.ErrorContains(t, err, "brave: decode response")
+}
+
+func TestBraveProvider_Search_EmptyResults(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		resp := braveResponse{}
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer srv.Close()
+
+	p := NewBraveProvider("key", srv.Client())
+	origURL := *braveSearchURL
+	u, _ := http.NewRequest("GET", srv.URL, nil)
+	*braveSearchURL = *u.URL
+	defer func() { *braveSearchURL = origURL }()
+
+	resp, err := p.Search(context.Background(), SearchRequest{Query: "test"})
+	require.NoError(t, err)
+	require.Empty(t, resp.Results)
+}