Add Reddit monitoring bot — backend, frontend, and Docker config

Python/FastAPI backend with PostgreSQL for collecting Reddit data via
public .json endpoints. React/Vite dashboard for analytics. Docker Compose
setup with API and worker services connecting to shared PostgreSQL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/models/metric_snapshot.py

@@ -0,0 +1,23 @@
+from datetime import datetime, timezone
+from sqlalchemy import Integer, Float, DateTime, ForeignKey, Index
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from backend.models.base import Base
+
+
+class MetricSnapshot(Base):
+    __tablename__ = "metric_snapshots"
+    __table_args__ = (
+        Index("ix_metric_snapshots_post_snapshot", "post_id", "snapshot_at"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    post_id: Mapped[int] = mapped_column(ForeignKey("posts.id"), nullable=False)
+    score: Mapped[int] = mapped_column(Integer, nullable=False)
+    num_comments: Mapped[int] = mapped_column(Integer, nullable=False)
+    upvote_ratio: Mapped[float | None] = mapped_column(Float)
+    snapshot_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+
+    post: Mapped["Post"] = relationship(back_populates="metric_snapshots")  # noqa: F821