Fix false "Plex fallback" banner during the first scan

history_coverage is only written at the end of an ingest, but
has_completion_data was reading from it. During the first scan the coverage
table is empty while watch_event already holds tens of thousands of Tautulli
rows, so the dashboard announced a fallback that had not happened and claimed
the rejection component was disabled when it was not.

The flag now comes from the events themselves — does any row carry a
percent_complete — which is true the moment Tautulli rows land and false for
Plex-only history. history_source falls back to the scan record when coverage
is absent, so it reads "tautulli" mid-scan instead of null.

Also: the banner named Plex as the source without checking, and now reports
whichever source is actually active, and stays quiet while a scan is running
since the counts are still moving.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GVbG48GAXfCZatcmX123Ra

tests/test_rules_and_api.py

@@ -182,3 +182,28 @@ def test_bulk_keep(client, scanned):
     assert r.status_code == 200
     assert r.get_json()["created"] == len(ids)
     assert client.get("/api/v1/keeps").get_json()["kept_items"] >= len(ids)
+
+
+def test_completion_flag_comes_from_events_not_the_coverage_table(client, scanned):
+    """history_coverage is written at the END of an ingest. Reading the flag from
+    it made the UI announce a Plex fallback mid-scan on every fresh database."""
+    from mediashelf import queries
+
+    assert queries.history_has_completion(scanned) is True
+
+    # simulate mid-scan: events present, coverage not yet written
+    scanned.execute("DELETE FROM history_coverage")
+    assert queries.history_has_completion(scanned) is True, \
+        "flag went false with events already ingested"
+    assert queries.active_history_source(scanned) == "tautulli", \
+        "source should fall back to the scan record when coverage is absent"
+
+    ov = client.get("/api/v1/stats/overview").get_json()
+    assert ov["has_completion_data"] is True
+    assert ov["history_source"] == "tautulli"
+
+
+def test_completion_flag_is_false_for_plex_only_history(scanned):
+    from mediashelf import queries
+    scanned.execute("UPDATE watch_event SET percent_complete = NULL")
+    assert queries.history_has_completion(scanned) is False