feat: allow editing frequency and target_value on habits

- Add frequency to UpdateHabitRequest (was immutable, now editable)
- Validate frequency + specific_days/dates coherence on update
- Keep type and is_negative immutable (they change entry semantics)
- Remove completion_rate references from swagger and README

docs/swagger.yaml

@@ -268,6 +268,10 @@ definitions:
         type: boolean
       description:
         type: string
+      frequency:
+        $ref: '#/definitions/value_objects.Frequency'
+      is_negative:
+        type: boolean
       name:
         type: string
       specific_dates:
@@ -1189,8 +1193,7 @@ paths:
       - system
   /stats/habits/{id}:
     get:
-      description: Get statistics for a specific habit including streaks and completion
-        rates
+      description: Get statistics for a specific habit including streaks and completions
       parameters:
       - description: Habit ID
         in: path