fix(alembic 0007): 改用 sa.Column 内嵌 ForeignKey 形式,避免列数不匹配

之前用独立 sa.ForeignKeyConstraint 报 ArgumentError,
改用 sa.Column(..., sa.ForeignKey(...), ...) 形式(
跟 model 里的 mapped_column 形式对齐)。

backend/alembic/versions/0007_article_reads.py

@@ -1,6 +1,6 @@
 """文章已读记录(per-user)
 
-- article_reads.user_id       BIGINT  FK users.id    ON DELETE CASCADE
+- article_reads.user_id       INTEGER       FK users.id    ON DELETE CASCADE
 - article_reads.article_id    BIGINT        FK articles.id ON DELETE CASCADE
 - article_reads.read_at       TIMESTAMPTZ   DEFAULT now()
 - 复合主键 (user_id, article_id) — 天然幂等
@@ -25,16 +25,25 @@ depends_on = None
 def upgrade() -> None:
     op.create_table(
         "article_reads",
-        sa.Column("user_id", sa.Integer, nullable=False),  # users.id 是 Integer
-        sa.Column("article_id", sa.BigInteger, nullable=False),  # articles.id 是 BigInteger
-        sa.Column("read_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.text("now()")),
+        sa.Column(
+            "user_id",
+            sa.Integer,
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "article_id",
+            sa.BigInteger,
+            sa.ForeignKey("articles.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "read_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("now()"),
+        ),
         sa.PrimaryKeyConstraint("user_id", "article_id", name="pk_article_reads"),
-        sa.ForeignKeyConstraint(
-            "user_id", ["users.id"], ondelete="CASCADE",
-        ),
-        sa.ForeignKeyConstraint(
-            "article_id", ["articles.id"], ondelete="CASCADE",
-        ),
     )
     op.create_index(
         "ix_article_reads_user_read_at",