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Drops the unused tag.namespace column from FC-1 — superseded by kind.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

@@ -40,6 +40,12 @@ class ImageRecord(Base):
     width: Mapped[int | None] = mapped_column(Integer, nullable=True)
     height: Mapped[int | None] = mapped_column(Integer, nullable=True)
 
+    # Integrity verification status. FC-2e populates this; FC-2a leaves rows at 'unknown'.
+    # Values: 'unknown' (default), 'ok', 'corrupt', 'failed_verification'.
+    integrity_status: Mapped[str] = mapped_column(
+        String(24), nullable=False, default="unknown", index=True
+    )
+
     # Thumbnail (populated by FC-2)
     thumbnail_path: Mapped[str | None] = mapped_column(Text, nullable=True)