feat: overlay stock price on trend charts with right Y axis

- New GET /api/market/prices/{ticker} endpoint serving OHLCV data from
  market_snapshots, deduped by bar_timestamp
- New useMarketPrices hook in frontend
- Trend chart now shows price (purple line) on a right Y axis ($)
  alongside trend metrics (%) on the left Y axis
- Custom tooltip formats price as dollars, metrics as percentages
- Price line uses connectNulls for days with missing bar data

services/api/app.py

@@ -465,6 +465,56 @@ async def list_trend_history(
     return results
 
 
+@app.get("/api/market/prices/{ticker}")
+async def get_market_prices(
+    ticker: str,
+    limit: int = Query(default=30, le=200),
+):
+    """Return historical close prices for a ticker from market_snapshots.
+
+    Each row has a bar_date (from the Polygon bar timestamp) and OHLCV data.
+    Ordered oldest-first for chart rendering.
+    """
+    ticker = ticker.upper()
+    rows = await pool.fetch(
+        """SELECT
+               captured_at,
+               (data->>'c')::float AS close,
+               (data->>'o')::float AS open,
+               (data->>'h')::float AS high,
+               (data->>'l')::float AS low,
+               (data->>'v')::float AS volume,
+               (data->>'t')::bigint AS bar_timestamp
+           FROM market_snapshots
+           WHERE ticker = $1 AND snapshot_type = 'bar'
+           ORDER BY captured_at ASC
+           LIMIT $2""",
+        ticker, limit,
+    )
+    results = []
+    seen_dates: set[str] = set()
+    for r in rows:
+        # Deduplicate by bar_timestamp (same day bar captured multiple times)
+        bar_ts = r["bar_timestamp"]
+        if bar_ts is None:
+            continue
+        date_key = str(bar_ts)
+        if date_key in seen_dates:
+            continue
+        seen_dates.add(date_key)
+        results.append({
+            "ticker": ticker,
+            "close": r["close"],
+            "open": r["open"],
+            "high": r["high"],
+            "low": r["low"],
+            "volume": r["volume"],
+            "bar_timestamp": bar_ts,
+            "captured_at": r["captured_at"].isoformat() if r["captured_at"] else None,
+        })
+    return results
+
+
 @app.get("/api/trends/{trend_id}")
 async def get_trend(trend_id: str):
     """Get a single trend summary by ID."""