perf(ml): batch the auto-apply sweeps' image_tag inserts (#3072)

Item 1 of #3072. Both sweeps issued a single-row pg_insert(image_tag)
from inside their per-image loop. Steady state that is nothing; a first
sweep over a back-catalogue is one round-trip per applied tag, tens of
thousands of them. Each chunk now collects its rows and writes them in
one statement.

The ticket suggested one insert per chunk PER TAG. A single multi-row
VALUES carries every tag at once, so it is one statement per chunk full
stop — and the sweeps already accumulate across all heads before they
commit, so nothing had to be restructured to allow it.

Not a new helper: wip_title.apply_wip_image_tags was already doing the
chunked ON CONFLICT DO NOTHING insert, so that shape is extracted to
services/image_tag_apply.insert_image_tags and all three writers share
it. The extraction deliberately leaves wip_title's pre-SELECT behind
rather than pulling it into the shared function — the sweeps don't need
it (their `skip` sets already exclude applied and rejected images) and
it exists only to produce an accurate count, which the sweeps also
compute themselves. So the shared primitive returns nothing: psycopg
reports rowcount -1 for a multi-row ON CONFLICT DO NOTHING insert, and a
count taken from the statement would be a lie rather than an
approximation.

Ordering note for the system-tag sweep: tag rows are now written after
that chunk's PresentationReview rows rather than interleaved before
them. Safe — PresentationReview FKs to image_record and tag, not to
image_tag.

Chunk size stays 5000: 5000 rows x 3 bound params = 15000, inside
Postgres' 65535-parameter ceiling with room to spare.

tests/test_image_tag_apply.py covers the primitive directly, since it is
now the single place three writers can be wrong at once — most
importantly that a re-run never restamps a hand-applied tag's source,
which would silently poison head training (it excludes the auto
sources).

Left alone: _insert_presentation_review is still per-row, and the
retract path still deletes per-row. Both operate on sets that are small
by construction, unlike the apply path.

Refs #3072

backend/app/services/wip_title.py

@@ -20,10 +20,10 @@ family gains one member.
 import re
 
 from sqlalchemy import select
-from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session
 
 from ..models.tag import WIP_SYSTEM_TAG, Tag, image_tag
+from .image_tag_apply import insert_image_tags
 
 # image_tag.source stamped on title-heuristic WIP tags — distinct from the other
 # apply sources so provenance stays legible and a future undo can target only these.
@@ -113,13 +113,9 @@ def apply_wip_image_tags(
         to_insert = [iid for iid in chunk if iid not in already]
         if not to_insert:
             continue
-        session.execute(
-            pg_insert(image_tag)
-            .values([
-                {"image_record_id": iid, "tag_id": tag_id, "source": source}
-                for iid in to_insert
-            ])
-            .on_conflict_do_nothing(index_elements=["image_record_id", "tag_id"])
-        )
+        insert_image_tags(session, [
+            {"image_record_id": iid, "tag_id": tag_id, "source": source}
+            for iid in to_insert
+        ])
         inserted += len(to_insert)
     return inserted