feat: ticker validation + SLV/USO/WEAT/CORN/TUR added to ETFs watchlist

Backend:
- GET /api/market/validate?symbol= — validates ticker against yfinance,
  returns {valid, name, price} or {valid: false, reason: 'helpful message'}
- Added SLV, USO, WEAT, CORN, TUR to ETFs WATCHLIST category

Frontend:
- validateTicker() async helper exported from useApi.ts
- InstrumentPicker (PatternLab): custom ticker field now validates before selecting
  Shows spinner while checking, red error message if not found on yfinance
- InstrumentLens (PatternExplorer): same validation on Go button + Enter key

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

frontend/src/pages/PatternLab.tsx

@@ -3,6 +3,7 @@ import {
   useRunPatternLab, useEvaluatePatternLab, useSaveLabPattern,
   usePatternLabRuns, useDeleteLabRun,
   useInstrumentScan, useEvaluateInstrumentScan,
+  validateTicker,
 } from '../hooks/useApi'
 import {
   FlaskConical, Play, CheckCircle2, XCircle, MinusCircle,
@@ -146,9 +147,11 @@ function OutcomeRow({ out }: { out: any }) {
 // ── Shared instrument picker ───────────────────────────────────────────────────
 
 function InstrumentPicker({ onSelect }: { onSelect: (ticker: string, name: string) => void }) {
-  const [search, setSearch] = useState('')
-  const [cat, setCat]       = useState<string | null>(null)
-  const [custom, setCustom] = useState('')
+  const [search, setSearch]         = useState('')
+  const [cat, setCat]               = useState<string | null>(null)
+  const [custom, setCustom]         = useState('')
+  const [validating, setValidating] = useState(false)
+  const [tickerError, setTickerError] = useState<string | null>(null)
 
   const seen = new Set<string>()
   const unique = INSTRUMENTS.filter(i => { if (seen.has(i.ticker)) return false; seen.add(i.ticker); return true })
@@ -162,6 +165,26 @@ function InstrumentPicker({ onSelect }: { onSelect: (ticker: string, name: strin
     return true
   })
 
+  const handleCustom = async () => {
+    const sym = custom.trim().toUpperCase()
+    if (!sym) return
+    setTickerError(null)
+    setValidating(true)
+    try {
+      const res = await validateTicker(sym)
+      if (res.valid) {
+        onSelect(sym, res.name || sym)
+        setCustom('')
+      } else {
+        setTickerError(res.reason || `Ticker '${sym}' not found`)
+      }
+    } catch {
+      setTickerError(`Could not validate '${sym}' — check your connection`)
+    } finally {
+      setValidating(false)
+    }
+  }
+
   return (
     <div className="space-y-2">
       {/* Category pills */}
@@ -186,10 +209,24 @@ function InstrumentPicker({ onSelect }: { onSelect: (ticker: string, name: strin
           <input value={search} onChange={e => setSearch(e.target.value)} placeholder="Search…"
             className="w-full pl-6 pr-2 py-1.5 text-xs bg-dark-700 border border-slate-700/50 rounded text-slate-200 placeholder-slate-600 focus:outline-none focus:border-violet-500" />
         </div>
-        <input value={custom} onChange={e => setCustom(e.target.value)}
-          onKeyDown={e => e.key === 'Enter' && custom.trim() && onSelect(custom.trim(), custom.trim())}
-          placeholder="Custom ticker"
-          className="w-28 px-2 py-1.5 text-xs bg-dark-700 border border-slate-700/50 rounded font-mono text-slate-200 placeholder-slate-600 focus:outline-none focus:border-violet-500" />
+        <div className="flex flex-col gap-1">
+          <div className="flex gap-1">
+            <input value={custom} onChange={e => { setCustom(e.target.value); setTickerError(null) }}
+              onKeyDown={e => e.key === 'Enter' && handleCustom()}
+              placeholder="Custom ticker"
+              className={clsx(
+                'w-28 px-2 py-1.5 text-xs bg-dark-700 border rounded font-mono text-slate-200 placeholder-slate-600 focus:outline-none uppercase',
+                tickerError ? 'border-red-500/70 focus:border-red-400' : 'border-slate-700/50 focus:border-violet-500'
+              )} />
+            <button onClick={handleCustom} disabled={!custom.trim() || validating}
+              className="px-2 py-1.5 text-xs bg-dark-700 border border-slate-700/50 rounded text-slate-400 hover:text-slate-200 hover:border-violet-500 disabled:opacity-40 transition-colors">
+              {validating ? <RefreshCw className="w-3 h-3 animate-spin" /> : '→'}
+            </button>
+          </div>
+          {tickerError && (
+            <p className="text-[9px] text-red-400 max-w-[140px] leading-tight">{tickerError}</p>
+          )}
+        </div>
       </div>
       {/* Grid */}
       <div className="grid grid-cols-3 gap-1 max-h-60 overflow-y-auto pr-1">