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>

app/services/watchlist_service.py

@@ -173,6 +173,9 @@ async def _enrich_entry(
         "dimensions": dims,
         "rr_ratio": setup.rr_ratio if setup else None,
         "rr_direction": setup.direction if setup else None,
+        # 12-1 cross-sectional momentum percentile (the top-pick selector); ticker-
+        # level, so any of the ticker's setups carries the same value.
+        "momentum_percentile": setup.momentum_percentile if setup else None,
         "sr_levels": sr_levels,
         "last_close": last_close,
         "change_pct": change_pct,