Schedule auto-discovery at 4 AM daily instead of every 23 hours

Replaced the rolling 23-hour check with a fixed-time scheduler that sleeps
until the next 4:00 AM, runs discovery for all users, then sleeps until the
following 4 AM. No longer reads last_discovery_run — just runs at the same
time every day.

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

backend/main.py

@@ -226,9 +226,15 @@ def on_startup():
         from sqlalchemy import text as _text
         from .services.discovery import schedule_discovery, get_discovery_progress
 
-        # Wait 5 minutes after startup before the first check so the app can
-        # finish initialising and existing enrichment tasks can settle.
-        _time.sleep(300)
+        def _seconds_until_4am():
+            now = _dt.now()
+            target = now.replace(hour=4, minute=0, second=0, microsecond=0)
+            if now >= target:
+                target += _td(days=1)
+            return (target - now).total_seconds()
+
+        # Sleep until the next 4 AM before doing anything
+        _time.sleep(_seconds_until_4am())
 
         while True:
             try:
@@ -236,28 +242,25 @@ def on_startup():
                 try:
                     rows = db.execute(_text("""
                         SELECT u.id AS user_id,
-                               COALESCE(us.discovery_regions, 'US,SE') AS discovery_regions,
-                               us.last_discovery_run
+                               COALESCE(us.discovery_regions, 'US,SE') AS discovery_regions
                         FROM users u
                         LEFT JOIN user_settings us ON u.id = us.user_id
                     """)).mappings().all()
 
                     for row in rows:
-                        last = row["last_discovery_run"]
-                        if last is None or (_dt.utcnow() - _dt.fromisoformat(str(last))) > _td(hours=23):
-                            uid = row["user_id"]
-                            # Skip if a manual sweep is already running
-                            prog = get_discovery_progress(uid)
-                            if prog and prog.get("running"):
-                                continue
-                            regions = [r.strip().upper() for r in (row["discovery_regions"] or "US,SE").split(",") if r.strip()]
-                            schedule_discovery(uid, regions)
+                        uid = row["user_id"]
+                        prog = get_discovery_progress(uid)
+                        if prog and prog.get("running"):
+                            continue
+                        regions = [r.strip().upper() for r in (row["discovery_regions"] or "US,SE").split(",") if r.strip()]
+                        schedule_discovery(uid, regions)
                 finally:
                     db.close()
             except Exception:
                 pass
 
-            _time.sleep(3600)  # check every hour, run if >23 h since last run
+            # Sleep until the next 4 AM
+            _time.sleep(_seconds_until_4am())
 
     threading.Thread(target=_auto_discovery_daemon, daemon=True).start()