Fix Garmin stats sync, add route merge/map/links, fix PR constraint

Garmin health: fix display_name=None when using stored OAuth tokens.
authenticate_garmin() now calls login(tokenstore=...) instead of
garth.loads() directly, so display_name is populated and get_user_summary
works. Also add avg_hr_day / max_hr_day from stats response.

Routes: add merge endpoint (POST /{id}/merge/{source}), delete endpoint.
Routes page: polyline SVG mini-map on each route card, merge UI with
confirmation, activity rows are now Links to the activity detail page.

Personal records: replace all-columns unique constraint with a partial
index (unique on current records only) to stop UniqueViolation crashes
when parallel workers deactivate the same PR.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/routes.py

@@ -176,6 +176,61 @@ async def route_activities(
     ]
 
 
+@router.post("/{route_id}/merge/{source_id}", response_model=RouteOut)
+async def merge_routes(
+    route_id: int,
+    source_id: int,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Move all activities from source route into route_id, then delete source route."""
+    from sqlalchemy import update
+
+    target = (await db.execute(
+        select(NamedRoute).where(NamedRoute.id == route_id, NamedRoute.user_id == current_user.id)
+    )).scalar_one_or_none()
+    source = (await db.execute(
+        select(NamedRoute).where(NamedRoute.id == source_id, NamedRoute.user_id == current_user.id)
+    )).scalar_one_or_none()
+    if not target or not source:
+        raise HTTPException(status_code=404, detail="Route not found")
+    if route_id == source_id:
+        raise HTTPException(status_code=400, detail="Cannot merge a route with itself")
+
+    await db.execute(
+        update(Activity)
+        .where(Activity.named_route_id == source_id, Activity.user_id == current_user.id)
+        .values(named_route_id=route_id)
+    )
+    await db.delete(source)
+    await db.commit()
+    await db.refresh(target)
+    return target
+
+
+@router.delete("/{route_id}")
+async def delete_route(
+    route_id: int,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    from sqlalchemy import update as sa_update
+    route = (await db.execute(
+        select(NamedRoute).where(NamedRoute.id == route_id, NamedRoute.user_id == current_user.id)
+    )).scalar_one_or_none()
+    if not route:
+        raise HTTPException(status_code=404, detail="Route not found")
+    # Unlink activities before deleting
+    await db.execute(
+        sa_update(Activity)
+        .where(Activity.named_route_id == route_id, Activity.user_id == current_user.id)
+        .values(named_route_id=None)
+    )
+    await db.delete(route)
+    await db.commit()
+    return {"status": "ok"}
+
+
 @router.post("/{route_id}/assign-activity")
 async def assign_activity_to_route(
     route_id: int,