feat(tags): TagService.merge guards + MergeResult + repoint stubs

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_tag_merge.py

@@ -3,6 +3,7 @@ import pytest
 from backend.app.models import Tag, TagKind, image_tag
 from backend.app.models.tag_allowlist import TagAllowlist
 from backend.app.services.tag_service import (
+    MergeResult,
     TagMergeConflict,
     TagService,
     TagValidationError,
@@ -76,3 +77,55 @@ async def test_will_alias_true_when_allowlisted(db):
     with pytest.raises(TagMergeConflict) as ei:
         await svc.rename(source.id, "Canon2")
     assert ei.value.will_alias is True
+
+
+@pytest.mark.asyncio
+async def test_merge_rejects_self_merge(db):
+    svc = TagService(db)
+    t = await svc.find_or_create("Solo", TagKind.general)
+    with pytest.raises(TagValidationError, match="into itself"):
+        await svc.merge(t.id, t.id)
+
+
+@pytest.mark.asyncio
+async def test_merge_rejects_missing_tag(db):
+    svc = TagService(db)
+    t = await svc.find_or_create("Real", TagKind.general)
+    with pytest.raises(TagValidationError, match="not found"):
+        await svc.merge(t.id, 999999)
+
+
+@pytest.mark.asyncio
+async def test_merge_rejects_cross_kind(db):
+    svc = TagService(db)
+    a = await svc.find_or_create("NameA", TagKind.general)
+    b = await svc.find_or_create("NameB", TagKind.artist)
+    with pytest.raises(TagValidationError, match="same kind"):
+        await svc.merge(a.id, b.id)
+
+
+@pytest.mark.asyncio
+async def test_merge_rejects_cross_fandom(db):
+    svc = TagService(db)
+    f1 = await svc.find_or_create("Fandom1", TagKind.fandom)
+    f2 = await svc.find_or_create("Fandom2", TagKind.fandom)
+    c1 = await svc.find_or_create("Char", TagKind.character, fandom_id=f1.id)
+    c2 = await svc.find_or_create("Char2", TagKind.character, fandom_id=f2.id)
+    with pytest.raises(TagValidationError, match="same kind and fandom"):
+        await svc.merge(c1.id, c2.id)
+
+
+@pytest.mark.asyncio
+async def test_merge_empty_tags_returns_result(db):
+    svc = TagService(db)
+    a = await svc.find_or_create("EmptyA", TagKind.general)
+    b = await svc.find_or_create("EmptyB", TagKind.general)
+    result = await svc.merge(a.id, b.id)
+    assert isinstance(result, MergeResult)
+    assert result.target_id == b.id
+    assert result.target_name == "EmptyB"
+    assert result.target_kind == "general"
+    assert result.merged_count == 0
+    assert result.alias_created is False
+    assert result.source_deleted is True
+    assert await db.get(Tag, a.id) is None