perf: SQLite WAL + 复合索引 + GZip + scheduler jitter + 构建缓存

backend/app/models/snapshot.py

@@ -1,5 +1,5 @@
 from datetime import datetime, timezone
-from sqlalchemy import Integer, Text, DateTime, ForeignKey
+from sqlalchemy import Index, Integer, Text, DateTime, ForeignKey, text
 from sqlalchemy.orm import mapped_column, Mapped
 from app.database import Base
 
@@ -11,3 +11,7 @@ class UpstreamRateSnapshot(Base):
     upstream_id: Mapped[int] = mapped_column(Integer, ForeignKey("upstreams.id", ondelete="CASCADE"), index=True)
     snapshot_json: Mapped[str] = mapped_column(Text, nullable=False)
     captured_at: Mapped[datetime] = mapped_column(DateTime, default=lambda: datetime.now(timezone.utc), index=True)
+
+    __table_args__ = (
+        Index("ix_snapshot_upstream_captured", "upstream_id", text("captured_at DESC")),
+    )