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/PatternExplorer.tsx

@@ -3,13 +3,14 @@ import { Link } from 'react-router-dom'
 import clsx from 'clsx'
 import {
   TreePine, Search, ChevronRight, ChevronDown,
-  TrendingUp, TrendingDown, Zap, BookOpen, ExternalLink,
+  TrendingUp, TrendingDown, Zap, BookOpen, ExternalLink, RefreshCw,
 } from 'lucide-react'
 import {
   useAllPatterns,
   usePatternTaxonomy,
   usePatternsByInstrument,
   useLastScores,
+  validateTicker,
 } from '../hooks/useApi'
 import { INSTRUMENTS, INSTRUMENT_CATEGORIES } from '../constants/instruments'
 
@@ -591,7 +592,9 @@ function InstrumentRow({ pattern, searchTerm }: { pattern: Pattern; searchTerm:
 function InstrumentLens({ patterns }: { patterns: Pattern[] }) {
   const [searchTerm, setSearchTerm]     = useState('')
   const [activeCategory, setActiveCategory] = useState<string | null>(null)
-  const [customInput, setCustomInput]   = useState('')
+  const [customInput, setCustomInput]     = useState('')
+  const [validating, setValidating]       = useState(false)
+  const [tickerError, setTickerError]     = useState<string | null>(null)
   const debounceRef = useRef<ReturnType<typeof setTimeout> | null>(null)
 
   // Selected ticker — either clicked from grid or typed
@@ -629,9 +632,24 @@ function InstrumentLens({ patterns }: { patterns: Pattern[] }) {
     )
   }, [uniqueInstruments, activeCategory, searchTerm])
 
-  const handleCustom = () => {
-    const t = customInput.trim()
-    if (t) { setSelectedTicker(t); setCustomInput('') }
+  const handleCustom = async () => {
+    const sym = customInput.trim().toUpperCase()
+    if (!sym) return
+    setTickerError(null)
+    setValidating(true)
+    try {
+      const res = await validateTicker(sym)
+      if (res.valid) {
+        setSelectedTicker(sym)
+        setCustomInput('')
+      } else {
+        setTickerError(res.reason || `Ticker '${sym}' not found`)
+      }
+    } catch {
+      setTickerError(`Could not validate '${sym}' — check your connection`)
+    } finally {
+      setValidating(false)
+    }
   }
 
   return (
@@ -660,12 +678,25 @@ function InstrumentLens({ patterns }: { patterns: Pattern[] }) {
             placeholder="Search instruments…"
             className="w-full pl-8 pr-3 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-blue-600" />
         </div>
-        <input type="text" value={customInput} onChange={e => setCustomInput(e.target.value)}
-          onKeyDown={e => e.key === 'Enter' && handleCustom()}
-          placeholder="Custom ticker…"
-          className="w-32 px-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-blue-600 font-mono" />
-        <button onClick={handleCustom}
-          className="px-2.5 py-1.5 text-xs bg-blue-700 hover:bg-blue-600 text-white rounded transition-colors">Go</button>
+        <div className="flex flex-col gap-1">
+          <div className="flex gap-1">
+            <input type="text" value={customInput}
+              onChange={e => { setCustomInput(e.target.value); setTickerError(null) }}
+              onKeyDown={e => e.key === 'Enter' && handleCustom()}
+              placeholder="Custom ticker…"
+              className={clsx(
+                'w-32 px-2 py-1.5 text-xs bg-dark-700 border rounded text-slate-200 placeholder-slate-600 focus:outline-none font-mono uppercase',
+                tickerError ? 'border-red-500/70 focus:border-red-400' : 'border-slate-700/50 focus:border-blue-600'
+              )} />
+            <button onClick={handleCustom} disabled={!customInput.trim() || validating}
+              className="px-2.5 py-1.5 text-xs bg-blue-700 hover:bg-blue-600 disabled:opacity-40 text-white rounded transition-colors flex items-center gap-1">
+              {validating ? <><RefreshCw className="w-3 h-3 animate-spin" /> …</> : 'Go'}
+            </button>
+          </div>
+          {tickerError && (
+            <p className="text-[9px] text-red-400 max-w-[200px] leading-tight">{tickerError}</p>
+          )}
+        </div>
         {selectedTicker && (
           <button onClick={() => setSelectedTicker('')}
             className="text-xs text-slate-500 hover:text-slate-300 transition-colors">Clear</button>