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

docs/swagger.yaml

@@ -384,8 +384,6 @@ definitions:
     type: object
   queries.HabitStatsDTO:
     properties:
-      completion_rate:
-        type: number
       completions_this_month:
         type: integer
       completions_this_week: