feat(activity): add --by-activity flag to break down spent time by activity type

- Load time entry activity names from Redmine enumeration.
- Aggregate hours per issue and per activity in fetch_issues_with_spent_time.
- Add --by-activity CLI flag and propagate it through report builder,
  summary, and all formatters (console, CSV, HTML, JSON, ODT).
- Keep backward-compatible 2-tuple input in build_grouped_report.
- Bump version to 1.8.0.

Closes #41

redmine_reporter/formatters/json.py

@@ -12,8 +12,9 @@ class JSONFormatter(Formatter):
         super().__init__()
 
     def format(self, rows: List[ReportRow]) -> str:
-        data = [
-            {
+        data = []
+        for r in rows:
+            item = {
                 "project": r["project"],
                 "version": r["version"],
                 "issue_id": r["issue_id"],
@@ -21,8 +22,10 @@ class JSONFormatter(Formatter):
                 "status": r["status_ru"],
                 "time": r["time_text"],
             }
-            for r in rows
-        ]
+            activities = r.get("activities")
+            if activities:
+                item["activities"] = activities
+            data.append(item)
         return json.dumps(data, ensure_ascii=False, indent=2)
 
     def save(self, rows: List[ReportRow], output_path: str) -> None: