feat: generate transaction records from snapshot diffs in seed script

Derive buy/sell transactions by comparing consecutive snapshots and
replace existing portfolio on re-run instead of skipping.

backend/scripts/seed_data.py

@@ -8,7 +8,7 @@ Requires: DATABASE_URL environment variable or default dev connection.
 """
 import sys
 import os
-from datetime import date
+from datetime import date, datetime
 from decimal import Decimal
 
 # Add backend to path
@@ -19,6 +19,7 @@ from app.core.database import SessionLocal
 from app.models.portfolio import (
     Portfolio, PortfolioType, Target, Holding,
     PortfolioSnapshot, SnapshotHolding,
+    Transaction, TransactionType,
 )
 from app.models.user import User
 
@@ -131,6 +132,17 @@ SNAPSHOTS = [
             {"ticker": "411060", "qty": 328, "price": Decimal("29605"), "value": Decimal("9710440")},
         ],
     },
+    {
+        "date": date(2026, 2, 16),
+        "total_assets": Decimal("62433665"),
+        "holdings": [
+            {"ticker": "069500", "qty": 16, "price": Decimal("81835"), "value": Decimal("1309360")},
+            {"ticker": "148070", "qty": 133, "price": Decimal("108290"), "value": Decimal("14402570")},
+            {"ticker": "284430", "qty": 1386, "price": Decimal("19250"), "value": Decimal("26680500")},
+            {"ticker": "360750", "qty": 385, "price": Decimal("24435"), "value": Decimal("9407475")},
+            {"ticker": "411060", "qty": 328, "price": Decimal("32420"), "value": Decimal("10633760")},
+        ],
+    },
 ]
 
 
@@ -142,14 +154,15 @@ def seed(db: Session):
         print("ERROR: No user found in database. Create a user first.")
         return
 
-    # Check if portfolio already exists
+    # Delete existing portfolio if present (cascade deletes related records)
     existing = db.query(Portfolio).filter(
         Portfolio.user_id == user.id,
         Portfolio.name == "연금 포트폴리오",
     ).first()
     if existing:
-        print(f"Portfolio '연금 포트폴리오' already exists (id={existing.id}). Skipping.")
-        return
+        db.delete(existing)
+        db.flush()
+        print(f"Deleted existing portfolio (id={existing.id})")
 
     # Create portfolio
     portfolio = Portfolio(
@@ -189,6 +202,45 @@ def seed(db: Session):
             ))
         print(f"  Snapshot {snap['date']}: {len(snap['holdings'])} holdings")
 
+    # Create transactions by comparing consecutive snapshots
+    tx_count = 0
+    for i, snap in enumerate(SNAPSHOTS):
+        current_holdings = {h["ticker"]: h for h in snap["holdings"]}
+
+        if i == 0:
+            # First snapshot: all holdings are initial buys
+            prev_holdings = {}
+        else:
+            prev_holdings = {h["ticker"]: h for h in SNAPSHOTS[i - 1]["holdings"]}
+
+        all_tickers = set(current_holdings.keys()) | set(prev_holdings.keys())
+        for ticker in all_tickers:
+            cur_qty = current_holdings[ticker]["qty"] if ticker in current_holdings else 0
+            prev_qty = prev_holdings[ticker]["qty"] if ticker in prev_holdings else 0
+            diff = cur_qty - prev_qty
+
+            if diff == 0:
+                continue
+
+            if diff > 0:
+                tx_type = TransactionType.BUY
+                price = current_holdings[ticker]["price"]
+            else:
+                tx_type = TransactionType.SELL
+                price = prev_holdings[ticker]["price"]
+
+            db.add(Transaction(
+                portfolio_id=portfolio.id,
+                ticker=ticker,
+                tx_type=tx_type,
+                quantity=abs(diff),
+                price=price,
+                executed_at=datetime.combine(snap["date"], datetime.min.time()),
+            ))
+            tx_count += 1
+
+    print(f"Created {tx_count} transactions from snapshot diffs")
+
     # Set current holdings from latest snapshot
     latest = SNAPSHOTS[-1]
     for h in latest["holdings"]: