fix(stats): the full-history ratio counts the requested channels on both sides

"Full history: done / requested" counted every subscribed channel whose
back-catalog was complete in the numerator - also the ones another user asked
for, or short enough to finish on their own - against only the channels this
user requested, so the Stats page read "342/329". The numerator now counts the
requested channels that are done; "channels without full history yet" still
counts every subscribed one.

backend/tests/test_my_status_deep.py

@@ -0,0 +1,33 @@
+"""The Stats page's "Full history: done / requested" ratio counts the SAME channels on both sides — the
+ones this user asked full history for. The numerator used to count every subscribed channel whose
+back-catalog was complete (another user had asked, or the channel was short), so the page read
+"342/329". "Channels without full history yet" still counts every subscribed channel. Needs the DB."""
+
+from app.models import Channel, Subscription
+
+
+def test_full_history_ratio_counts_only_the_requested_channels(db, make_user, as_user):
+    user = make_user("deep@example.com")
+    db.add_all(
+        [
+            Channel(id="UCreqdone", title="Requested, done", backfill_done=True),
+            Channel(id="UCreqopen", title="Requested, pending", backfill_done=False),
+            Channel(id="UCdone", title="Not requested, done anyway", backfill_done=True),
+            Channel(id="UCopen", title="Not requested, pending", backfill_done=False),
+        ]
+    )
+    db.flush()
+    db.add_all(
+        [
+            Subscription(user_id=user.id, channel_id="UCreqdone", deep_requested=True),
+            Subscription(user_id=user.id, channel_id="UCreqopen", deep_requested=True),
+            Subscription(user_id=user.id, channel_id="UCdone"),
+            Subscription(user_id=user.id, channel_id="UCopen"),
+        ]
+    )
+    db.commit()
+
+    body = as_user(user).get("/api/sync/my-status").json()
+    assert body["channels_deep_requested"] == 2
+    assert body["channels_deep_done"] == 1  # never more than it is out of
+    assert body["channels_deep_pending"] == 2  # subscribed channels without full history yet