refactor(upstream): replace upstream account type with apikey, auto-append /antigravity

Upstream accounts now use the standard APIKey type instead of a dedicated
upstream type. GetBaseURL() and new GetGeminiBaseURL() automatically append
/antigravity for Antigravity platform APIKey accounts, eliminating the need
for separate upstream forwarding methods.

- Remove ForwardUpstream, ForwardUpstreamGemini, testUpstreamConnection
- Remove upstream branch guards in Forward/ForwardGemini/TestConnection
- Add migration 052 to convert existing upstream accounts to apikey
- Update frontend CreateAccountModal to create apikey type
- Add unit tests for GetBaseURL and GetGeminiBaseURL

backend/internal/service/account_base_url_test.go

@@ -0,0 +1,160 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+)
+
+func TestGetBaseURL(t *testing.T) {
+	tests := []struct {
+		name     string
+		account  Account
+		expected string
+	}{
+		{
+			name: "non-apikey type returns empty",
+			account: Account{
+				Type:     AccountTypeOAuth,
+				Platform: PlatformAnthropic,
+			},
+			expected: "",
+		},
+		{
+			name: "apikey without base_url returns default anthropic",
+			account: Account{
+				Type:        AccountTypeAPIKey,
+				Platform:    PlatformAnthropic,
+				Credentials: map[string]any{},
+			},
+			expected: "https://api.anthropic.com",
+		},
+		{
+			name: "apikey with custom base_url",
+			account: Account{
+				Type:        AccountTypeAPIKey,
+				Platform:    PlatformAnthropic,
+				Credentials: map[string]any{"base_url": "https://custom.example.com"},
+			},
+			expected: "https://custom.example.com",
+		},
+		{
+			name: "antigravity apikey auto-appends /antigravity",
+			account: Account{
+				Type:        AccountTypeAPIKey,
+				Platform:    PlatformAntigravity,
+				Credentials: map[string]any{"base_url": "https://upstream.example.com"},
+			},
+			expected: "https://upstream.example.com/antigravity",
+		},
+		{
+			name: "antigravity apikey trims trailing slash before appending",
+			account: Account{
+				Type:        AccountTypeAPIKey,
+				Platform:    PlatformAntigravity,
+				Credentials: map[string]any{"base_url": "https://upstream.example.com/"},
+			},
+			expected: "https://upstream.example.com/antigravity",
+		},
+		{
+			name: "antigravity non-apikey returns empty",
+			account: Account{
+				Type:        AccountTypeOAuth,
+				Platform:    PlatformAntigravity,
+				Credentials: map[string]any{"base_url": "https://upstream.example.com"},
+			},
+			expected: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := tt.account.GetBaseURL()
+			if result != tt.expected {
+				t.Errorf("GetBaseURL() = %q, want %q", result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestGetGeminiBaseURL(t *testing.T) {
+	const defaultGeminiURL = "https://generativelanguage.googleapis.com"
+
+	tests := []struct {
+		name     string
+		account  Account
+		expected string
+	}{
+		{
+			name: "apikey without base_url returns default",
+			account: Account{
+				Type:        AccountTypeAPIKey,
+				Platform:    PlatformGemini,
+				Credentials: map[string]any{},
+			},
+			expected: defaultGeminiURL,
+		},
+		{
+			name: "apikey with custom base_url",
+			account: Account{
+				Type:        AccountTypeAPIKey,
+				Platform:    PlatformGemini,
+				Credentials: map[string]any{"base_url": "https://custom-gemini.example.com"},
+			},
+			expected: "https://custom-gemini.example.com",
+		},
+		{
+			name: "antigravity apikey auto-appends /antigravity",
+			account: Account{
+				Type:        AccountTypeAPIKey,
+				Platform:    PlatformAntigravity,
+				Credentials: map[string]any{"base_url": "https://upstream.example.com"},
+			},
+			expected: "https://upstream.example.com/antigravity",
+		},
+		{
+			name: "antigravity apikey trims trailing slash",
+			account: Account{
+				Type:        AccountTypeAPIKey,
+				Platform:    PlatformAntigravity,
+				Credentials: map[string]any{"base_url": "https://upstream.example.com/"},
+			},
+			expected: "https://upstream.example.com/antigravity",
+		},
+		{
+			name: "antigravity oauth does NOT append /antigravity",
+			account: Account{
+				Type:        AccountTypeOAuth,
+				Platform:    PlatformAntigravity,
+				Credentials: map[string]any{"base_url": "https://upstream.example.com"},
+			},
+			expected: "https://upstream.example.com",
+		},
+		{
+			name: "oauth without base_url returns default",
+			account: Account{
+				Type:        AccountTypeOAuth,
+				Platform:    PlatformAntigravity,
+				Credentials: map[string]any{},
+			},
+			expected: defaultGeminiURL,
+		},
+		{
+			name: "nil credentials returns default",
+			account: Account{
+				Type:     AccountTypeAPIKey,
+				Platform: PlatformGemini,
+			},
+			expected: defaultGeminiURL,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := tt.account.GetGeminiBaseURL(defaultGeminiURL)
+			if result != tt.expected {
+				t.Errorf("GetGeminiBaseURL() = %q, want %q", result, tt.expected)
+			}
+		})
+	}
+}