fix: the native path never reported tier-gated posts (milestone 387 step A1)

`make_run_stats` has always declared `tier_gated_count`, and
DownloadDetailModal has always rendered it. gallery-dl populated it;
`ingest_core._result` did not — it built run_stats with six keys and let
the seventh default to 0, while the very same walk counted gated posts
into `gated_skipped` and spent the number on a log line.

So on Patreon, SubscribeStar and pixiv — the three platforms we now own —
the Downloads modal read "Tier-gated: 0" for a walk that skipped N
paywalled posts. A creator we've lost access to was indistinguishable
from a creator who stopped posting. Migrating Patreon off gallery-dl is
what dropped the signal.

Pass the count through, and tick it in the live-progress payload too, so
a long backfill on an inaccessible creator explains itself while it runs
rather than only at finalization. ActiveDownloadsPanel renders it only
when non-zero, coloured 'info' to match the severity FailingSourcesCard
already assigns tier_limited — this is not a failure.

Tests assert the run_stats key the UI actually reads rather than the
ingester's internal counter, so the guard tracks the property and not a
name. Falsification is structural: `make_run_stats` defaults the key to
0, so both assertions fail against the pre-fix call.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LNXXULQDjVZmbuNa2G9mD9

backend/app/services/ingest_core.py

@@ -245,6 +245,12 @@ class Ingester:
                     per_item_failures=errors,
                     quarantined_count=quarantined,
                     dead_lettered_count=dead_lettered,
+                    # #874 follow-up: the native path counted gated posts but
+                    # never reported them, so DownloadDetailModal's "Tier-gated"
+                    # field read 0 on every native walk while gallery-dl's read
+                    # true. A paywalled creator was indistinguishable from a
+                    # silent one.
+                    tier_gated_count=gated_skipped,
                 ),
             )
 
@@ -464,6 +470,10 @@ class Ingester:
                         "errors": errors,
                         "quarantined": quarantined,
                         "posts": posts_processed,
+                        # Ticks during the walk, not only at finalization: a
+                        # deep backfill on a creator we've lost access to is
+                        # otherwise a long run of zeros with no explanation.
+                        "gated": gated_skipped,
                     })
 
                 if early_out: