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Drops the unused tag.namespace column from FC-1 — superseded by kind.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

@@ -0,0 +1,41 @@
+"""ImportTask — a single source file to import as part of a batch.
+
+State machine: pending -> queued -> processing -> complete | skipped | failed.
+Workers crashing mid-task leave rows in 'processing'; the recovery sweep
+(tasks/maintenance.py::recover_interrupted_tasks) re-queues rows that have
+been processing longer than the stuck-task threshold.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import BigInteger, DateTime, ForeignKey, Integer, String, Text, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from .base import Base
+
+
+class ImportTask(Base):
+    __tablename__ = "import_task"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    batch_id: Mapped[int] = mapped_column(
+        ForeignKey("import_batch.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+
+    source_path: Mapped[str] = mapped_column(Text, nullable=False)
+    task_type: Mapped[str] = mapped_column(String(16), nullable=False)  # media|archive
+    status: Mapped[str] = mapped_column(String(16), nullable=False, default="pending", index=True)
+
+    result_image_id: Mapped[int | None] = mapped_column(
+        ForeignKey("image_record.id", ondelete="SET NULL"), nullable=True
+    )
+    error: Mapped[str | None] = mapped_column(Text, nullable=True)
+    size_bytes: Mapped[int | None] = mapped_column(BigInteger, nullable=True)
+
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+    started_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    finished_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+
+    batch = relationship("ImportBatch", back_populates="tasks")