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/models/user.py

@@ -91,6 +91,7 @@ class Activity(Base):
     id = Column(Integer, primary_key=True)
     user_id = Column(Integer, ForeignKey("users.id"), nullable=False, index=True)
     name = Column(String(256), nullable=False)
+    original_name = Column(String(256), nullable=True)  # Garmin/import title, kept when user renames
     sport_type = Column(String(64), nullable=False)
     start_time = Column(DateTime(timezone=True), nullable=False, index=True)
     end_time = Column(DateTime(timezone=True), nullable=True)