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/database.py

@@ -97,6 +97,7 @@ def init_db():
             name TEXT,
             added_at TEXT DEFAULT (datetime('now'))
         )""",
+        "ALTER TABLE market_watchlist ADD COLUMN asset_class TEXT DEFAULT 'custom'",
     ]:
         try:
             c.execute(_sql)
@@ -2318,18 +2319,18 @@ def remove_watchlist_ticker(ticker: str) -> bool:
 def get_market_custom_tickers() -> List[Dict]:
     conn = get_conn()
     rows = conn.execute(
-        "SELECT ticker, name, added_at FROM market_watchlist ORDER BY added_at ASC"
+        "SELECT ticker, name, asset_class, added_at FROM market_watchlist ORDER BY added_at ASC"
     ).fetchall()
     conn.close()
     return [dict(r) for r in rows]
 
 
-def add_market_custom_ticker(ticker: str, name: str = "") -> bool:
+def add_market_custom_ticker(ticker: str, name: str = "", asset_class: str = "custom") -> bool:
     conn = get_conn()
     try:
         conn.execute(
-            "INSERT OR IGNORE INTO market_watchlist (ticker, name) VALUES (?, ?)",
-            (ticker.upper(), name),
+            "INSERT OR IGNORE INTO market_watchlist (ticker, name, asset_class) VALUES (?, ?, ?)",
+            (ticker.upper(), name, asset_class),
         )
         inserted = conn.total_changes > 0
         conn.commit()