feat: add rate-limit retry with exponential backoff in Gitea API client

Add automatic retry logic to doRequest for HTTP 429 responses. Uses
Retry-After header when present, otherwise exponential backoff
(1s, 2s, 4s). Respects context cancellation during waits. Defaults
to 3 max retries with 1s base delay. Includes 7 new tests covering
retry success, exhaustion, Retry-After header, context cancellation,
non-429 errors, and backoff calculation.

Closes leeworks-agents/gitea-mobile#132

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/gitea/client_test.go

@@ -1087,3 +1087,161 @@ func TestListAllPullRequests_Pagination(t *testing.T) {
 		t.Error("page 2: HasMore should be false")
 	}
 }
+
+func TestDoRequest_RateLimitRetry(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts <= 2 {
+			w.Header().Set("Retry-After", "0")
+			w.WriteHeader(http.StatusTooManyRequests)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `[{"username":"test-org"}]`)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 3
+	c.baseRetryDelay = 1 * time.Millisecond // Fast for tests.
+
+	resp, err := c.doRequest(context.Background(), "test-token", "GET", "/user/orgs", nil)
+	if err != nil {
+		t.Fatalf("expected success after retries, got: %v", err)
+	}
+	resp.Body.Close()
+
+	if attempts != 3 {
+		t.Errorf("expected 3 attempts, got %d", attempts)
+	}
+}
+
+func TestDoRequest_RateLimitExhausted(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusTooManyRequests)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 2
+	c.baseRetryDelay = 1 * time.Millisecond
+
+	_, err := c.doRequest(context.Background(), "test-token", "GET", "/user/orgs", nil)
+	if err == nil {
+		t.Fatal("expected error after exhausting retries")
+	}
+	if !strings.Contains(err.Error(), "rate limit exceeded") {
+		t.Errorf("expected rate limit error, got: %v", err)
+	}
+}
+
+func TestDoRequest_RateLimitWithRetryAfterHeader(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts == 1 {
+			w.Header().Set("Retry-After", "1")
+			w.WriteHeader(http.StatusTooManyRequests)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `[]`)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 3
+	c.baseRetryDelay = 1 * time.Millisecond
+
+	start := time.Now()
+	resp, err := c.doRequest(context.Background(), "test-token", "GET", "/user/orgs", nil)
+	elapsed := time.Since(start)
+	if err != nil {
+		t.Fatalf("expected success, got: %v", err)
+	}
+	resp.Body.Close()
+
+	// Retry-After: 1 means 1 second delay.
+	if elapsed < 900*time.Millisecond {
+		t.Errorf("expected at least ~1s delay from Retry-After header, got %v", elapsed)
+	}
+}
+
+func TestDoRequest_RateLimitCancelledContext(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Retry-After", "60")
+		w.WriteHeader(http.StatusTooManyRequests)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 3
+	c.baseRetryDelay = 1 * time.Millisecond
+
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+
+	_, err := c.doRequest(ctx, "test-token", "GET", "/user/orgs", nil)
+	if err == nil {
+		t.Fatal("expected error from cancelled context")
+	}
+}
+
+func TestDoRequest_NonRateLimitErrorNotRetried(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		w.WriteHeader(http.StatusForbidden)
+		fmt.Fprint(w, `{"message":"forbidden"}`)
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL)
+	c.maxRetries = 3
+	c.baseRetryDelay = 1 * time.Millisecond
+
+	_, err := c.doRequest(context.Background(), "test-token", "GET", "/user/orgs", nil)
+	if err == nil {
+		t.Fatal("expected error for 403")
+	}
+	if attempts != 1 {
+		t.Errorf("expected only 1 attempt for non-429 error, got %d", attempts)
+	}
+}
+
+func TestRetryDelay_WithRetryAfterHeader(t *testing.T) {
+	c := NewClient("https://example.com")
+	c.baseRetryDelay = 1 * time.Second
+
+	resp := &http.Response{Header: http.Header{}}
+	resp.Header.Set("Retry-After", "5")
+
+	delay := c.retryDelay(resp, 0)
+	if delay != 5*time.Second {
+		t.Errorf("expected 5s from Retry-After, got %v", delay)
+	}
+}
+
+func TestRetryDelay_ExponentialBackoff(t *testing.T) {
+	c := NewClient("https://example.com")
+	c.baseRetryDelay = 1 * time.Second
+
+	resp := &http.Response{Header: http.Header{}}
+
+	tests := []struct {
+		attempt int
+		want    time.Duration
+	}{
+		{0, 1 * time.Second},
+		{1, 2 * time.Second},
+		{2, 4 * time.Second},
+	}
+
+	for _, tt := range tests {
+		delay := c.retryDelay(resp, tt.attempt)
+		if delay != tt.want {
+			t.Errorf("attempt %d: got %v, want %v", tt.attempt, delay, tt.want)
+		}
+	}
+}