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/services/garmin_connect_sync.py

@@ -46,15 +46,17 @@ def authenticate_garmin(email: str, password_enc: str, token_store: Optional[str
     """
     import garminconnect
 
-    # Try stored OAuth token first (garth auto-refreshes access token on use)
+    # Try stored OAuth token first.
+    # Must call login(tokenstore=...) rather than garth.loads() directly so that
+    # garmin.display_name is populated — it's required by get_user_summary() and
+    # several other endpoints.  Without it every stats call silently returns None.
     if token_store:
         try:
             garmin = garminconnect.Garmin(
                 email=email, password=decrypt_password(password_enc)
             )
-            garmin.garth.loads(token_store)
-            garmin.get_full_name()          # lightweight request; triggers refresh if needed
-            return garmin, None             # tokens still valid
+            garmin.login(tokenstore=token_store)
+            return garmin, None
         except Exception as exc:
             logger.info("Garmin token invalid (%s), re-authenticating", exc)
 
@@ -234,6 +236,8 @@ def _parse_day(stats, sleep_data, hrv_data) -> dict:
 
     if stats:
         _set(row, "resting_hr",      stats.get("restingHeartRate"))
+        _set(row, "avg_hr_day",      stats.get("averageHeartRate"))
+        _set(row, "max_hr_day",      stats.get("maxHeartRate"))
         _set(row, "steps",           stats.get("totalSteps"))
         _set(row, "floors_climbed",  stats.get("floorsAscended"))
         _set(row, "avg_stress",      stats.get("averageStressLevel"))