feat: ticker search, watchlist momentum column, alpha vs S&P 500

Three usability fixes:

1. Global ticker search in the sidebar (TickerSearch) — typeahead over the
   tracked universe that opens a ticker's detail page without adding it to the
   watchlist. Also wired into the mobile nav.

2. Watchlist table shows the ticker's 12-1 momentum percentile (the top-pick
   selector) instead of the noisy full S/R-level list. Enriched from the setup
   already loaded in watchlist_service._enrich_entry — no extra query.

3. Alpha vs the S&P 500 on paper trades (open + closed). New benchmark_prices
   table + benchmark_service store SPY daily closes (a standalone series, not a
   Ticker, so it never enters the scanner / momentum ranking / rankings) via a
   new daily-pipeline step. paper_trade_service computes per-trade
   benchmark_return / alpha_pct / alpha_usd over each holding period; the open-
   trades table, dashboard, and closed-trades panel surface per-trade and total
   alpha. The list read path never makes a provider call.

Deploy: alembic upgrade head, then run the benchmark/daily job once to populate
SPY closes (alpha shows "—" until then).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/watchlist/WatchlistTable.tsx

@@ -42,7 +42,7 @@ export function WatchlistTable({ entries }: WatchlistTableProps) {
             <th className="px-4 py-3">Dimensions</th>
             <th className="px-4 py-3">R:R</th>
             <th className="px-4 py-3">Direction</th>
-            <th className="px-4 py-3">S/R Levels</th>
+            <th className="px-4 py-3">Momentum</th>
             <th className="px-4 py-3"></th>
           </tr>
         </thead>
@@ -114,15 +114,9 @@ export function WatchlistTable({ entries }: WatchlistTableProps) {
                   <span className="text-gray-500">—</span>
                 )}
               </td>
-              <td className="px-4 py-3.5">
-                {entry.sr_levels.length > 0 ? (
-                  <div className="flex flex-wrap gap-1">
-                    {entry.sr_levels.map((level, i) => (
-                      <span key={i} className={`text-xs ${level.type === 'support' ? 'text-emerald-400' : 'text-red-400'}`}>
-                        {formatPrice(level.price_level)}
-                      </span>
-                    ))}
-                  </div>
+              <td className="px-4 py-3.5 num text-gray-200">
+                {entry.momentum_percentile !== null ? (
+                  `${Math.round(entry.momentum_percentile)}%ile`
                 ) : (
                   <span className="text-gray-500">—</span>
                 )}