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/domain/services/streak.go

@@ -0,0 +1,98 @@
+package services
+
+import (
+	"time"
+
+	"apocapoc-api/internal/domain/entities"
+	"apocapoc-api/internal/shared/utils"
+)
+
+type StreakResult struct {
+	Current int
+	Longest int
+}
+
+func CalculateStreaks(entries []*entities.HabitEntry, habit *entities.Habit, now time.Time) StreakResult {
+	entryMap := buildEntryMap(entries)
+	scheduled := allScheduledDates(habit, now)
+
+	if len(scheduled) == 0 {
+		return StreakResult{}
+	}
+
+	today := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, time.UTC)
+	longest := 0
+	current := 0
+
+	for _, d := range scheduled {
+		if IsDaySuccessful(d.Format("2006-01-02"), entryMap, habit) {
+			current++
+			if current > longest {
+				longest = current
+			}
+		} else if d.Equal(today) && !habit.IsNegative {
+			continue
+		} else {
+			current = 0
+		}
+	}
+
+	return StreakResult{Current: current, Longest: longest}
+}
+
+func buildEntryMap(entries []*entities.HabitEntry) map[string]*entities.HabitEntry {
+	m := make(map[string]*entities.HabitEntry)
+	for _, e := range entries {
+		m[e.ScheduledDate.Format("2006-01-02")] = e
+	}
+	return m
+}
+
+func allScheduledDates(habit *entities.Habit, now time.Time) []time.Time {
+	start := time.Date(habit.CreatedAt.Year(), habit.CreatedAt.Month(), habit.CreatedAt.Day(), 0, 0, 0, 0, time.UTC)
+	today := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, time.UTC)
+	freq := string(habit.Frequency)
+
+	var dates []time.Time
+	for d := start; !d.After(today); d = d.AddDate(0, 0, 1) {
+		if utils.ShouldAppearToday(freq, habit.SpecificDays, habit.SpecificDates, d) {
+			dates = append(dates, d)
+		}
+	}
+	return dates
+}
+
+func IsDaySuccessful(dateStr string, entryMap map[string]*entities.HabitEntry, habit *entities.Habit) bool {
+	entry, hasEntry := entryMap[dateStr]
+	habitType := string(habit.Type)
+
+	if !habit.IsNegative {
+		if !hasEntry {
+			return false
+		}
+		if habit.TargetValue != nil && entry.Value != nil {
+			return *entry.Value >= *habit.TargetValue
+		}
+		return true
+	}
+
+	if habitType == "VALUE" && habit.TargetValue != nil {
+		if !hasEntry {
+			return false
+		}
+		if entry.Value != nil {
+			return *entry.Value <= *habit.TargetValue
+		}
+		return false
+	}
+
+	if !hasEntry {
+		return true
+	}
+
+	if habit.TargetValue != nil && entry.Value != nil {
+		return *entry.Value <= *habit.TargetValue
+	}
+
+	return false
+}