feat(tags): non-mutating merge preview + admin dry_run (#8a)

Cluster B, milestone #99. TagService.merge_preview(source, target) computes the
same counts the apply produces (rule 93 parity) without mutating: images_moving
(source links the apply UPDATEs), images_already_on_target (links it drops),
source_total, series_pages, will_alias (_keep_as_alias), a kind/fandom
compatible flag (surfaced, not raised, so the UI can warn), and up to 6
thumbnails of the moving images. The admin /tags/<dest>/merge route gains a
dry_run flag returning the preview JSON.

Tests: preview moving-count == apply merged_count (parity), incompatible flagged
without raising, self/missing raise, admin dry_run returns preview + no mutation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XCUHUGQLrBrkgyk1t49kpX

tests/test_api_admin.py

@@ -270,6 +270,41 @@ async def test_tag_merge_succeeds_for_same_kind(client, db):
     assert body["result"]["target_id"] == dest_id
 
 
+@pytest.mark.asyncio
+async def test_tag_merge_dry_run_previews_without_mutating(client, db):
+    from backend.app.models import ImageRecord
+    from backend.app.models.tag import image_tag
+
+    src = Tag(name="dry-src", kind=TagKind.general)
+    dest = Tag(name="dry-dest", kind=TagKind.general)
+    db.add_all([src, dest])
+    await db.flush()
+    img = ImageRecord(
+        path="/images/dry.jpg", sha256="dry" + "0" * 61, size_bytes=1,
+        mime="image/jpeg", width=1, height=1,
+        origin="imported_filesystem", integrity_status="unknown",
+    )
+    db.add(img)
+    await db.flush()
+    await db.execute(image_tag.insert().values(
+        image_record_id=img.id, tag_id=src.id, source="manual",
+    ))
+    src_id, dest_id = src.id, dest.id
+    await db.commit()
+
+    resp = await client.post(
+        f"/api/admin/tags/{dest_id}/merge",
+        json={"source_id": src_id, "dry_run": True},
+    )
+    assert resp.status_code == 200
+    p = (await resp.get_json())["preview"]
+    assert p["compatible"] is True
+    assert p["images_moving"] == 1
+    assert p["source_total"] == 1
+    # No mutation: source still exists.
+    assert await db.get(Tag, src_id) is not None
+
+
 @pytest.mark.asyncio
 async def test_tag_merge_rejects_self_merge(client, db):
     t = Tag(name="x", kind=TagKind.general)