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/layout/MobileNav.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import { NavLink } from 'react-router-dom';
 import { useAuthStore } from '../../stores/authStore';
+import TickerSearch from './TickerSearch';
 
 const navItems = [
   { to: '/', label: 'Overview', end: true },
@@ -46,6 +47,9 @@ export default function MobileNav() {
         }`}
       >
         <nav className="px-3 py-2 space-y-1">
+          <div className="pb-2">
+            <TickerSearch onNavigate={() => setOpen(false)} />
+          </div>
           {navItems.map(({ to, label, end }) => (
             <NavLink
               key={to}