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.json

@@ -1164,7 +1164,7 @@
                         "BearerAuth": []
                     }
                 ],
-                "description": "Get statistics for a specific habit including streaks and completion rates",
+                "description": "Get statistics for a specific habit including streaks and completions",
                 "produces": [
                     "application/json"
                 ],
@@ -1792,6 +1792,9 @@
                 "description": {
                     "type": "string"
                 },
+                "frequency": {
+                    "$ref": "#/definitions/value_objects.Frequency"
+                },
                 "name": {
                     "type": "string"
                 },