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Drops the unused tag.namespace column from FC-1 — superseded by kind.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

backend/app/models/import_settings.py

@@ -0,0 +1,28 @@
+"""ImportSettings — single-row table holding the importer's tunable knobs.
+
+Enforced as a single row via a CHECK (id = 1) constraint. The application
+always SELECTs id=1 and never inserts/deletes after the initial migration.
+"""
+
+from sqlalchemy import Boolean, CheckConstraint, Float, Integer, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class ImportSettings(Base):
+    __tablename__ = "import_settings"
+    __table_args__ = (CheckConstraint("id = 1", name="ck_import_settings_singleton"),)
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    import_scan_path: Mapped[str] = mapped_column(Text, nullable=False, default="/import")
+
+    min_width: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    min_height: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
+    skip_transparent: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    transparency_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.9)
+
+    skip_single_color: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    single_color_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.95)
+    single_color_tolerance: Mapped[int] = mapped_column(Integer, nullable=False, default=30)