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/main.py

@@ -50,6 +50,24 @@ async def init_db():
     except Exception as e:
         print(f"Column migration skipped: {e}")
 
+    # Replace the all-columns unique constraint on personal_records with a partial
+    # index (only current records must be unique per user/sport/distance).
+    # The old constraint also covered is_current_record=False rows, causing
+    # UniqueViolation crashes when multiple workers deactivate the same PR.
+    try:
+        async with engine.begin() as conn:
+            await conn.execute(text(
+                "ALTER TABLE personal_records "
+                "DROP CONSTRAINT IF EXISTS uq_pr_current"
+            ))
+            await conn.execute(text(
+                "CREATE UNIQUE INDEX IF NOT EXISTS uq_pr_current_active "
+                "ON personal_records (user_id, sport_type, distance_m) "
+                "WHERE is_current_record = true"
+            ))
+    except Exception as e:
+        print(f"PR constraint migration skipped: {e}")
+
     # Seed admin user (only if password is configured)
     if not settings.admin_password:
         print("ADMIN_PASSWORD not set - skipping admin user seed")