Improve portfolio chart with historical price-based data points

Chart now plots weekly data points from first purchase to today using
candle/history price data, giving an accurate view of portfolio value
over time rather than just at purchase dates.

Backend seeds up to 365 days of daily close prices from CoinGecko as
synthetic OHLC candles, refreshing stale entries older than 31 days.

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

btc-portfolio/backend/app/services/candles.py

@@ -1,10 +1,10 @@
 import logging
-from datetime import datetime, timezone, date as dt_date
+from datetime import datetime, timezone, timedelta, date as dt_date
 
 from sqlalchemy.orm import Session
 
 from ..models import OHLCCandle
-from .btc import get_btc_ohlc_eur, aggregate_to_daily
+from .btc import get_btc_ohlc_eur, aggregate_to_daily, get_btc_history_eur
 
 logger = logging.getLogger(__name__)
 
@@ -30,10 +30,54 @@ def seed_candles(db: Session) -> None:
     logger.info("Candle seed: stored %d daily candles (%s → %s).", len(rows), min(daily.keys()), max(daily.keys()))
 
 
+def seed_historical_prices(db: Session) -> None:
+    """Backfill up to 365 days of daily close prices from CoinGecko market_chart.
+    Uses previous day's close as each day's open to produce red/green candles.
+    Clears entries older than 31 days on each run so the data stays fresh.
+    Real OHLC entries (last 30 days) are never touched.
+    """
+    raw = get_btc_history_eur()
+    if not raw:
+        logger.warning("Historical price seed: CoinGecko returned no data.")
+        return
+
+    prices = {}
+    for ts_ms, price in raw:
+        date = datetime.fromtimestamp(ts_ms / 1000, tz=timezone.utc).strftime("%Y-%m-%d")
+        prices[date] = price
+
+    # Remove stale historical entries (older than 31 days) so they get re-seeded with current data
+    cutoff = (datetime.now(tz=timezone.utc) - timedelta(days=31)).strftime("%Y-%m-%d")
+    db.query(OHLCCandle).filter(OHLCCandle.date < cutoff).delete()
+    db.commit()
+
+    existing = {c.date for c in db.query(OHLCCandle).all()}
+
+    new_rows = []
+    prev_close = None
+    for date, close in sorted(prices.items()):
+        if date in existing:
+            prev_close = close
+            continue
+        open_ = prev_close if prev_close is not None else close
+        high = max(open_, close)
+        low = min(open_, close)
+        new_rows.append(OHLCCandle(date=date, open=open_, high=high, low=low, close=close))
+        prev_close = close
+
+    if new_rows:
+        db.add_all(new_rows)
+        db.commit()
+        logger.info("Historical price seed: stored %d daily entries (%s → %s).", len(new_rows), new_rows[0].date, new_rows[-1].date)
+
+
 def refresh_latest_candles(db: Session) -> None:
     """Add any missing candles up to today. Seeds the DB if empty.
     Also detects and replaces coarse (>2-day gap) legacy data from a previous days=365 seed.
     """
+    # Always backfill historical prices for dates not yet in DB (no-op once populated)
+    seed_historical_prices(db)
+
     # Sparse-data detection: if existing candles have >2-day gaps, wipe and re-seed
     first_two = db.query(OHLCCandle).order_by(OHLCCandle.date.asc()).limit(2).all()
     if len(first_two) == 2: