feat(attachments): PostAttachment model + 0009 (table + batch counter)

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

backend/app/models/post_attachment.py

@@ -0,0 +1,47 @@
+"""PostAttachment — a non-art file preserved from a post.
+
+Art images become ImageRecords; everything else a post contained
+(archives, .exe, .pdf, ...) is captured here so nothing is lost.
+post_id is nullable (set only when an adjacent sidecar yields a Post);
+artist_id mirrors the canonical attribution model (FC-2d-vii-c). Both
+FKs are SET NULL so deleting a Post/Artist never deletes the preserved
+binary row.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import (
+    BigInteger,
+    DateTime,
+    ForeignKey,
+    Integer,
+    String,
+    Text,
+    func,
+)
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class PostAttachment(Base):
+    __tablename__ = "post_attachment"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    post_id: Mapped[int | None] = mapped_column(
+        ForeignKey("post.id", ondelete="SET NULL"), nullable=True, index=True
+    )
+    artist_id: Mapped[int | None] = mapped_column(
+        ForeignKey("artist.id", ondelete="SET NULL"), nullable=True, index=True
+    )
+    sha256: Mapped[str] = mapped_column(
+        String(64), nullable=False, unique=True, index=True
+    )
+    path: Mapped[str] = mapped_column(Text, nullable=False)
+    original_filename: Mapped[str] = mapped_column(Text, nullable=False)
+    ext: Mapped[str] = mapped_column(String(32), nullable=False)
+    mime: Mapped[str | None] = mapped_column(String(128), nullable=True)
+    size_bytes: Mapped[int] = mapped_column(BigInteger, nullable=False)
+    captured_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )