Fix sync_lookback_days actually controlling the sync window

Activities: lookback_days was ignored once last_sync_at was set (since
always took priority). Now lookback_days always sets the window; -1 is
all-time on first sync then incremental.

Wellness: lookback_days was never passed to sync_wellness at all —
hardcoded 90-day cap regardless of settings. Fixed by adding lookback_days
param and wiring it through from the Celery task.

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

backend/app/services/garmin_connect_sync.py

@@ -74,9 +74,9 @@ def sync_activities(garmin, user_id: int, since: Optional[datetime],
     List activities from Garmin Connect, skip any already in the DB, download
     FIT ZIPs for new ones, and queue them for processing.
 
-    On first sync (since=None) the start date is determined by lookback_days:
-      -1 → full history back to 2010; N → today minus N days.
-    On incremental syncs fetches from one day before last_sync_at.
+    lookback_days controls the start date on every sync:
+      -1  → full history back to 2010 on first sync, then incremental (since-1d)
+      N   → always look back N days from today (dedup prevents re-downloading)
     Returns the number of new activities queued.
     """
     import time
@@ -84,10 +84,9 @@ def sync_activities(garmin, user_id: int, since: Optional[datetime],
     from app.models.user import Activity
     from sqlalchemy import select, func
 
-    if since:
-        start_date = (since - timedelta(days=1)).date()
-    elif lookback_days == -1:
-        start_date = date(2010, 1, 1)
+    if lookback_days == -1:
+        # All-time: full pull on first sync, incremental thereafter
+        start_date = (since - timedelta(days=1)).date() if since else date(2010, 1, 1)
     else:
         start_date = date.today() - timedelta(days=max(lookback_days, 1))
     end_date = date.today()
@@ -173,16 +172,23 @@ def sync_activities(garmin, user_id: int, since: Optional[datetime],
 
 # ── Wellness sync ─────────────────────────────────────────────────────────────
 
-def sync_wellness(garmin, user_id: int, since: Optional[datetime], db) -> int:
+def sync_wellness(garmin, user_id: int, since: Optional[datetime], db,
+                  lookback_days: int = 90) -> int:
     """
     Fetch daily stats / sleep / HRV from the Garmin Connect JSON API for each
-    day since `since` and upsert into health_metrics.
+    day in the window and upsert into health_metrics.
+
+    lookback_days controls the window on every sync:
+      -1 → full history back to 2010 on first sync, then incremental (since-1d)
+      N  → always cover the last N days (upsert is safe to re-run)
     Returns the number of days upserted.
     """
     from sqlalchemy import text
 
-    # First sync: 90 days of wellness history; incremental: from last sync
-    start_date = since.date() if since else (date.today() - timedelta(days=90))
+    if lookback_days == -1:
+        start_date = (since - timedelta(days=1)).date() if since else date(2010, 1, 1)
+    else:
+        start_date = date.today() - timedelta(days=max(lookback_days, 1))
     days = (date.today() - start_date).days + 1
     processed = 0