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/main.py

@@ -56,6 +56,9 @@ async def init_db():
             await conn.execute(text(
                 "ALTER TABLE activities ADD COLUMN IF NOT EXISTS moving_time_s FLOAT"
             ))
+            await conn.execute(text(
+                "ALTER TABLE activities ADD COLUMN IF NOT EXISTS original_name VARCHAR(256)"
+            ))
     except Exception as e:
         print(f"activities.moving_time_s column migration skipped: {e}")