Add pagination support to GET /api/v1/habits endpoint

- Create pagination package with Params and Response structs
- Add FindActiveByUserIDWithPagination and CountActiveByUserID methods to HabitRepository interface
- Implement pagination in SQLite repository using LIMIT and OFFSET
- Update GetUserHabitsHandler to support optional pagination parameters
- Modify HTTP endpoint to parse 'page' and 'page_size' query params (default: page=1, page_size=50, max=100)
- Add GetUserHabitsResponse DTO with pagination metadata
- Maintain backward compatibility - endpoint works with and without pagination params
- Add comprehensive tests for pagination logic in repository, handler, and pagination package
- Update all mock repositories to implement new pagination methods

internal/shared/pagination/pagination.go

@@ -0,0 +1,54 @@
+package pagination
+
+type Params struct {
+	Page     int
+	PageSize int
+}
+
+func NewParams(page, pageSize int) Params {
+	if page < 1 {
+		page = 1
+	}
+	if pageSize < 1 {
+		pageSize = 50
+	}
+	if pageSize > 100 {
+		pageSize = 100
+	}
+	return Params{
+		Page:     page,
+		PageSize: pageSize,
+	}
+}
+
+func (p Params) Offset() int {
+	return (p.Page - 1) * p.PageSize
+}
+
+func (p Params) Limit() int {
+	return p.PageSize
+}
+
+type Response struct {
+	Page       int `json:"page"`
+	PageSize   int `json:"page_size"`
+	TotalItems int `json:"total_items"`
+	TotalPages int `json:"total_pages"`
+}
+
+func NewResponse(params Params, totalItems int) Response {
+	totalPages := totalItems / params.PageSize
+	if totalItems%params.PageSize > 0 {
+		totalPages++
+	}
+	if totalPages < 1 {
+		totalPages = 1
+	}
+
+	return Response{
+		Page:       params.Page,
+		PageSize:   params.PageSize,
+		TotalItems: totalItems,
+		TotalPages: totalPages,
+	}
+}

internal/shared/pagination/pagination_test.go

@@ -0,0 +1,100 @@
+package pagination
+
+import "testing"
+
+func TestNewParams(t *testing.T) {
+	tests := []struct {
+		name         string
+		page         int
+		pageSize     int
+		expectedPage int
+		expectedSize int
+	}{
+		{"valid params", 1, 20, 1, 20},
+		{"valid params page 2", 2, 50, 2, 50},
+		{"page less than 1 defaults to 1", 0, 20, 1, 20},
+		{"negative page defaults to 1", -5, 20, 1, 20},
+		{"pageSize less than 1 defaults to 50", 1, 0, 1, 50},
+		{"negative pageSize defaults to 50", 1, -10, 1, 50},
+		{"pageSize greater than 100 caps at 100", 1, 200, 1, 100},
+		{"page 100", 1, 101, 1, 100},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			params := NewParams(tt.page, tt.pageSize)
+			if params.Page != tt.expectedPage {
+				t.Errorf("Page = %d, want %d", params.Page, tt.expectedPage)
+			}
+			if params.PageSize != tt.expectedSize {
+				t.Errorf("PageSize = %d, want %d", params.PageSize, tt.expectedSize)
+			}
+		})
+	}
+}
+
+func TestParamsOffset(t *testing.T) {
+	tests := []struct {
+		name           string
+		page           int
+		pageSize       int
+		expectedOffset int
+	}{
+		{"first page", 1, 20, 0},
+		{"second page", 2, 20, 20},
+		{"third page", 3, 20, 40},
+		{"page 10 size 50", 10, 50, 450},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			params := Params{Page: tt.page, PageSize: tt.pageSize}
+			offset := params.Offset()
+			if offset != tt.expectedOffset {
+				t.Errorf("Offset() = %d, want %d", offset, tt.expectedOffset)
+			}
+		})
+	}
+}
+
+func TestParamsLimit(t *testing.T) {
+	params := Params{Page: 1, PageSize: 25}
+	if params.Limit() != 25 {
+		t.Errorf("Limit() = %d, want 25", params.Limit())
+	}
+}
+
+func TestNewResponse(t *testing.T) {
+	tests := []struct {
+		name          string
+		params        Params
+		totalItems    int
+		expectedPages int
+		expectedTotal int
+	}{
+		{"exact division", Params{1, 20}, 100, 5, 100},
+		{"with remainder", Params{1, 20}, 105, 6, 105},
+		{"less than page size", Params{1, 20}, 15, 1, 15},
+		{"zero items", Params{1, 20}, 0, 1, 0},
+		{"one item", Params{1, 20}, 1, 1, 1},
+		{"large dataset", Params{1, 50}, 1000, 20, 1000},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			response := NewResponse(tt.params, tt.totalItems)
+			if response.Page != tt.params.Page {
+				t.Errorf("Page = %d, want %d", response.Page, tt.params.Page)
+			}
+			if response.PageSize != tt.params.PageSize {
+				t.Errorf("PageSize = %d, want %d", response.PageSize, tt.params.PageSize)
+			}
+			if response.TotalPages != tt.expectedPages {
+				t.Errorf("TotalPages = %d, want %d", response.TotalPages, tt.expectedPages)
+			}
+			if response.TotalItems != tt.expectedTotal {
+				t.Errorf("TotalItems = %d, want %d", response.TotalItems, tt.expectedTotal)
+			}
+		})
+	}
+}