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/worker/snapshot.py

@@ -0,0 +1,47 @@
+import logging
+from datetime import datetime, timezone, timedelta
+
+from sqlalchemy import select, create_engine
+from sqlalchemy.orm import sessionmaker
+
+from backend.config import settings
+from backend.models.post import Post
+from backend.models.metric_snapshot import MetricSnapshot
+from backend.models.subreddit import MonitoredSubreddit
+
+logger = logging.getLogger(__name__)
+
+_engine = create_engine(settings.database_url_sync, pool_size=2, pool_recycle=3600)
+SyncSession = sessionmaker(_engine)
+
+
+def take_metric_snapshots():
+    """Snapshot current metrics for recent posts."""
+    now = datetime.now(timezone.utc)
+
+    with SyncSession() as db:
+        # Posts < 48h old: snapshot every run (every 30 min)
+        cutoff_recent = now - timedelta(hours=48)
+        stmt = (
+            select(Post.id, Post.score, Post.num_comments, Post.upvote_ratio)
+            .join(MonitoredSubreddit)
+            .where(
+                MonitoredSubreddit.is_active == True,  # noqa: E712
+                Post.created_utc >= cutoff_recent,
+            )
+        )
+        result = db.execute(stmt)
+        snapshots = []
+        for post_id, score, num_comments, upvote_ratio in result:
+            snapshots.append(MetricSnapshot(
+                post_id=post_id,
+                score=score,
+                num_comments=num_comments,
+                upvote_ratio=upvote_ratio,
+                snapshot_at=now,
+            ))
+
+        if snapshots:
+            db.add_all(snapshots)
+            db.commit()
+            logger.info(f"Took {len(snapshots)} metric snapshots")