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.json

@@ -1970,9 +1970,6 @@
         "queries.HabitStatsDTO": {
             "type": "object",
             "properties": {
-                "completion_rate": {
-                    "type": "number"
-                },
                 "completions_this_month": {
                     "type": "integer"
                 },