fix: custom tickers now placed in their natural asset-class tab

- Detect yfinance quote_type (CURRENCY→forex, FUTURE→energy, INDEX→indices,
  ETF→etfs, EQUITY→equities) when adding a custom ticker and persist it in
  market_watchlist.asset_class
- get_all_quotes() merges custom tickers into their proper group (e.g. EURUSD=X
  appears under Forex) instead of always under a separate "Custom" group
- "Custom" tab only shows tickers whose type couldn't be detected
- Add market_watchlist.asset_class migration; ensure backtest_lab_runs and
  market_watchlist are always created at init_db() time

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

backend/services/data_fetcher.py

@@ -137,20 +137,23 @@ def get_all_quotes() -> Dict[str, List[Dict[str, Any]]]:
                 quotes.append(q)
         result[asset_class] = quotes
 
-    # User-added custom tickers
+    # User-added custom tickers — placed in their detected asset class group
     try:
         from services.database import get_market_custom_tickers
         custom_entries = get_market_custom_tickers()
-        if custom_entries:
-            custom_quotes = []
-            for entry in custom_entries:
-                q = get_quote(entry["ticker"])
-                if q:
-                    q["name"] = entry["name"] or entry["ticker"]
-                    q["asset_class"] = "custom"
-                    custom_quotes.append(q)
-            if custom_quotes:
-                result["custom"] = custom_quotes
+        for entry in (custom_entries or []):
+            q = get_quote(entry["ticker"])
+            if not q:
+                continue
+            q["name"] = entry["name"] or entry["ticker"]
+            grp = entry.get("asset_class") or "custom"
+            q["asset_class"] = grp
+            if grp not in result:
+                result[grp] = []
+            # avoid duplicates (ticker already in built-in group)
+            existing_symbols = {x["symbol"] for x in result[grp]}
+            if q["symbol"] not in existing_symbols:
+                result[grp].append(q)
     except Exception:
         pass