Add official US and European collection and calendar storage

Collect official RSS and Atom summaries, preserve historical captures, and poll macro releases with bounded cadence and Retry-After handling. Separate news collectors from the economic calendar worker.

Track statistical periods, publication stages, first observations, revisions, and current official series. Include explicit migration files and isolated archive, parsing, and import tests; no migration or import is executed by this commit.

Keep dependency declarations and lockfile together for PDF archives and the upcoming captcha integration. Verify the staged snapshot with 90 focused tests.

backend/alembic/versions/20261024_a8c1d4e7f230_calendar_periods.py

@@ -0,0 +1,47 @@
+"""日历统计期、发布阶段及当前官方历史序列。
+
+Revision ID: a8c1d4e7f230
+Revises: f7a9b3d5e210
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "a8c1d4e7f230"
+down_revision = "f7a9b3d5e210"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("economic_releases", sa.Column("period", sa.String(10)))
+    op.add_column("economic_releases", sa.Column("stage", sa.String(16)))
+    op.add_column("sources", sa.Column("collector", sa.String(16), nullable=False, server_default="news"))
+    sources = sa.table("sources", *[sa.column(k, t) for k, t in [
+        ("code", sa.String), ("name", sa.String), ("list_url", sa.String), ("collector", sa.String),
+        ("enabled", sa.Boolean), ("interval_seconds", sa.Integer), ("backfill_hours", sa.Integer),
+        ("consecutive_failures", sa.Integer),
+    ]])
+    op.bulk_insert(sources, [{"code": "umich", "name": "密歇根大学·经济日历", "collector": "calendar",
+                             "list_url": "https://www.sca.isr.umich.edu/", "enabled": True,
+                             "interval_seconds": 3600, "backfill_hours": 0, "consecutive_failures": 0}])
+    op.create_table(
+        "economic_series_values",
+        sa.Column("id", sa.Integer, primary_key=True),
+        sa.Column("source", sa.String(16), nullable=False),
+        sa.Column("indicator", sa.String(48), nullable=False),
+        sa.Column("period", sa.String(10), nullable=False),
+        sa.Column("stage", sa.String(16), nullable=False),
+        sa.Column("value", sa.String(64), nullable=False),
+        sa.Column("captured_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("raw_response_id", sa.BigInteger, sa.ForeignKey("raw_responses.id"), nullable=False),
+        sa.UniqueConstraint("source", "indicator", "period", "stage"),
+    )
+
+
+def downgrade() -> None:
+    # 已有原始响应时外键拒绝删除来源，不级联删除用户数据。
+    op.execute("DELETE FROM sources WHERE code = 'umich'")
+    op.drop_table("economic_series_values")
+    op.drop_column("sources", "collector")
+    op.drop_column("economic_releases", "stage")
+    op.drop_column("economic_releases", "period")