refactor: extract streak calculation to domain service

Move streak logic from application query to a dedicated domain service,
making it reusable and properly tested for all habit configurations.

- Support streaks for all habit types (boolean, counter, value) combined
  with positive/negative and optional target values
- Handle weekly habits with specific days (streak counts scheduled days)
- Today completed counts toward streak; not yet completed doesn't break it
- Calculate current and longest streak in a single forward pass
- Remove completion_rate from stats (not a useful metric for habits)
- Add comprehensive unit tests covering all 10 type combinations

internal/application/queries/get_habit_stats.go

@@ -6,18 +6,18 @@ import (
 
 	"apocapoc-api/internal/domain/entities"
 	"apocapoc-api/internal/domain/repositories"
+	"apocapoc-api/internal/domain/services"
 	"apocapoc-api/internal/shared/errors"
 )
 
 type HabitStatsDTO struct {
-	HabitID              string  `json:"habit_id"`
-	HabitName            string  `json:"habit_name"`
-	TotalCompletions     int     `json:"total_completions"`
-	CurrentStreak        int     `json:"current_streak"`
-	LongestStreak        int     `json:"longest_streak"`
-	CompletionRate       float64 `json:"completion_rate"`
-	CompletionsThisWeek  int     `json:"completions_this_week"`
-	CompletionsThisMonth int     `json:"completions_this_month"`
+	HabitID              string `json:"habit_id"`
+	HabitName            string `json:"habit_name"`
+	TotalCompletions     int    `json:"total_completions"`
+	CurrentStreak        int    `json:"current_streak"`
+	LongestStreak        int    `json:"longest_streak"`
+	CompletionsThisWeek  int    `json:"completions_this_week"`
+	CompletionsThisMonth int    `json:"completions_this_month"`
 }
 
 type GetHabitStatsQuery struct {
@@ -60,102 +60,21 @@ func (h *GetHabitStatsHandler) Handle(ctx context.Context, query GetHabitStatsQu
 		HabitName: habit.Name,
 	}
 
-	if len(entries) == 0 {
+	if len(entries) == 0 && !habit.IsNegative {
 		return stats, nil
 	}
 
 	stats.TotalCompletions = len(entries)
-	stats.CurrentStreak = calculateCurrentStreak(entries)
-	stats.LongestStreak = calculateLongestStreak(entries)
-	stats.CompletionRate = calculateCompletionRate(entries, habit.CreatedAt)
 	stats.CompletionsThisWeek = countCompletionsInPeriod(entries, 7)
 	stats.CompletionsThisMonth = countCompletionsInPeriod(entries, 30)
 
+	streaks := services.CalculateStreaks(entries, habit, time.Now().UTC())
+	stats.CurrentStreak = streaks.Current
+	stats.LongestStreak = streaks.Longest
+
 	return stats, nil
 }
 
-func calculateCurrentStreak(entries []*entities.HabitEntry) int {
-	if len(entries) == 0 {
-		return 0
-	}
-
-	dateMap := make(map[string]bool)
-	for _, entry := range entries {
-		dateStr := entry.ScheduledDate.Format("2006-01-02")
-		dateMap[dateStr] = true
-	}
-
-	streak := 0
-	currentDate := time.Now().UTC()
-
-	for {
-		dateStr := currentDate.Format("2006-01-02")
-		if !dateMap[dateStr] {
-			break
-		}
-		streak++
-		currentDate = currentDate.AddDate(0, 0, -1)
-	}
-
-	return streak
-}
-
-func calculateLongestStreak(entries []*entities.HabitEntry) int {
-	if len(entries) == 0 {
-		return 0
-	}
-
-	dateMap := make(map[string]bool)
-	var dates []time.Time
-	for _, entry := range entries {
-		date := time.Date(entry.ScheduledDate.Year(), entry.ScheduledDate.Month(), entry.ScheduledDate.Day(), 0, 0, 0, 0, time.UTC)
-		dateStr := date.Format("2006-01-02")
-		if !dateMap[dateStr] {
-			dateMap[dateStr] = true
-			dates = append(dates, date)
-		}
-	}
-
-	if len(dates) == 0 {
-		return 0
-	}
-
-	longestStreak := 1
-	currentStreak := 1
-
-	for i := 1; i < len(dates); i++ {
-		diff := dates[i].Sub(dates[i-1]).Hours() / 24
-		if diff == 1 {
-			currentStreak++
-			if currentStreak > longestStreak {
-				longestStreak = currentStreak
-			}
-		} else {
-			currentStreak = 1
-		}
-	}
-
-	return longestStreak
-}
-
-func calculateCompletionRate(entries []*entities.HabitEntry, createdAt time.Time) float64 {
-	if len(entries) == 0 {
-		return 0
-	}
-
-	daysSinceCreation := int(time.Since(createdAt).Hours() / 24)
-	if daysSinceCreation == 0 {
-		daysSinceCreation = 1
-	}
-
-	rate := float64(len(entries)) / float64(daysSinceCreation) * 100
-	if rate > 100 {
-		rate = 100
-	}
-
-	return rate
-}
-
 func countCompletionsInPeriod(entries []*entities.HabitEntry, days int) int {
 	cutoff := time.Now().UTC().AddDate(0, 0, -days)
 	count := 0