feat(fc2a): schema migration 0002 — tag kinds + fandom hierarchy + import lifecycle

Adds Tag.kind enum (artist/character/fandom/general/series/archive/post/meta/rating),
Tag.fandom_id FK with CHECK constraint (only valid for kind='character'), and a
kind-aware uniqueness index so the same name can exist across kinds and the same
character name can exist in different fandoms.

Adds ImportBatch + ImportTask state-machine tables for scan tracking, plus a
single-row ImportSettings table (CHECK id=1) holding the importer's filter knobs.

Adds image_record.integrity_status column (defaults to 'unknown'); FC-2e
populates this via the integrity verifier.

Drops the unused tag.namespace column from FC-1 — superseded by kind.

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

tests/test_migration_0002.py

@@ -0,0 +1,56 @@
+"""Smoke test for migration 0002: confirms model classes import and the
+tag-kind uniqueness rule shape is correct.
+"""
+
+from backend.app.models import (
+    Base,
+    ImportBatch,
+    ImportSettings,
+    ImportTask,
+    Tag,
+    TagKind,
+)
+
+
+def test_new_tables_registered():
+    expected = {"import_batch", "import_task", "import_settings"}
+    assert expected.issubset(Base.metadata.tables.keys())
+
+
+def test_tag_has_kind_and_fandom_id():
+    cols = {c.name for c in Tag.__table__.columns}
+    assert "kind" in cols
+    assert "fandom_id" in cols
+    assert "namespace" not in cols
+
+
+def test_tag_kind_enum_values():
+    expected = {
+        "artist",
+        "character",
+        "fandom",
+        "general",
+        "series",
+        "archive",
+        "post",
+        "meta",
+        "rating",
+    }
+    assert {k.value for k in TagKind} == expected
+
+
+def test_image_record_has_integrity_status():
+    from backend.app.models import ImageRecord
+    cols = {c.name for c in ImageRecord.__table__.columns}
+    assert "integrity_status" in cols
+
+
+def test_import_task_has_state_columns():
+    cols = {c.name for c in ImportTask.__table__.columns}
+    for required in ("batch_id", "source_path", "task_type", "status", "result_image_id"):
+        assert required in cols
+
+
+def test_import_settings_singleton_constraint():
+    constraints = {c.name for c in ImportSettings.__table__.constraints}
+    assert "ck_import_settings_singleton" in constraints