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

internal/infrastructure/http/habit_integration_test.go

@@ -89,6 +89,7 @@ func TestHabitCRUDFlow(t *testing.T) {
 		reqBody := UpdateHabitRequest{
 			Name:        "Morning Exercise",
 			Description: "Updated description",
+			Frequency:   "DAILY",
 		}
 
 		rr := makeRequest(t, *ts.Router, "PUT", "/api/v1/habits/"+habitID, reqBody, token)