feat: rename activities; keep original Garmin title as a tag

Add Activity.original_name (migrated via init_db ALTER). The rename
endpoint stores the import title the first time a user renames and
clears it if renamed back. Activity detail page gets inline rename
(pencil) UI and shows the original title beneath the new name; the
activities list shows it as a small tag too.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/activities.py

@@ -16,6 +16,7 @@ router = APIRouter()
 class ActivitySummary(BaseModel):
     id: int
     name: str
+    original_name: Optional[str] = None
     sport_type: str
     start_time: datetime
     distance_m: Optional[float]
@@ -368,9 +369,23 @@ async def rename_activity(
     if not activity:
         raise HTTPException(status_code=404, detail="Activity not found")
 
-    activity.name = body.get("name", activity.name)
+    new_name = (body.get("name") or "").strip()
+    if not new_name:
+        raise HTTPException(status_code=400, detail="Name cannot be empty")
+
+    # Preserve the original import title the first time the user renames, so the
+    # UI can still show it as a tag. Subsequent renames keep that same original.
+    if activity.original_name is None and new_name != activity.name:
+        activity.original_name = activity.name
+
+    activity.name = new_name
+
+    # If the user renames back to the original title, drop the tag.
+    if activity.original_name == activity.name:
+        activity.original_name = None
+
     await db.commit()
-    return {"id": activity_id, "name": activity.name}
+    return {"id": activity_id, "name": activity.name, "original_name": activity.original_name}
 
 
 @router.delete("/{activity_id}", status_code=204)