fix: use -inf sentinel in transition debounce so first emit fires on fresh VMs

The transition debounce used ``0.0`` as the "never-emitted" sentinel. On
GitHub Actions runners where ``time.monotonic()`` starts near zero (it
measures seconds since VM boot), ``now - 0.0`` can fall inside the 60s
debounce window — silently suppressing the very first emit and breaking
the four offline-sweep tests that rely on seeing a notification row.

Tests passed on local dev boxes (hours of uptime → monotonic in the
thousands) but failed on CI every time.

Fix: sentinel becomes ``float("-inf")`` so the first emit always clears
the threshold regardless of host uptime. Added a regression test that
monkeypatches ``time.monotonic`` to 5.0 to lock the behaviour in.

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

Author: Sbussiso

backend/app/api/notifications.py

@@ -174,10 +174,16 @@ def create_notification(
 def _should_emit_transition(kind: str, entity_id: str, direction: str) -> bool:
     """Return True if we haven't emitted this (kind,entity,direction)
     recently.  Updates the timestamp on a positive return so subsequent
-    calls within the debounce window are suppressed."""
+    calls within the debounce window are suppressed.
+
+    The sentinel for "never emitted before" is ``-inf``, not ``0.0`` —
+    otherwise on a freshly-booted host (e.g. GitHub Actions runners,
+    where ``time.monotonic()`` starts near zero) the very first emit
+    can fall inside the debounce window and get silently dropped.
+    """
     key = (kind, entity_id, direction)
     now = _time.monotonic()
-    last = _transition_debounce.get(key, 0.0)
+    last = _transition_debounce.get(key, float("-inf"))
     if now - last < _TRANSITION_DEBOUNCE_SECONDS:
         return False
     _transition_debounce[key] = now

backend/tests/test_notifications.py

@@ -298,6 +298,19 @@ def test_transition_different_cameras_not_debounced(db):
     assert db.query(Notification).count() == 2
 
 
+def test_transition_emits_on_fresh_boot_low_monotonic(db, monkeypatch):
+    # Regression: GitHub Actions runners can have a very small ``time.monotonic()``
+    # value (freshly-booted VM), which would collide with the 60s debounce window
+    # if the "never-emitted" sentinel was 0.0.  Simulate that here and make sure
+    # the first emit still fires.
+    monkeypatch.setattr("app.api.notifications._time.monotonic", lambda: 5.0)
+    emit_camera_transition(
+        db, camera_id="cam_fresh_boot", org_id="org_test123",
+        display_name="Fresh Boot", new_status="offline",
+    )
+    assert db.query(Notification).count() == 1
+
+
 # ── Offline sweep ─────────────────────────────────────────────────
 
 def _make_node(db, *, node_id, org_id, status, last_seen_minutes_ago, name=None):