feat: SQL Explorer with PostgreSQL schema browser and pre-built queries

The SQL Explorer was querying Trino which has zero tables. Rewrote to
use PostgreSQL directly:

Backend:
- GET /api/analytics/pg-schema: returns all public tables with column
  names, types, and nullability from information_schema
- POST /api/analytics/pg-query: read-only SQL execution against
  PostgreSQL with SELECT-only enforcement, auto LIMIT, and descriptive
  error messages for syntax/table/query errors

Frontend:
- Schema browser shows all PostgreSQL tables with columns and types
- Click a table name → generates SELECT * FROM table LIMIT 100
- Pre-built Queries section with 12 seeded queries covering companies,
  recommendations, trends, market prices, documents, global events,
  trading decisions, ingestion health, reserve pool, sector exposure
- User-saved queries shown separately with delete buttons
- Chart builder, Monaco editor, and save functionality preserved

Migration 021: seeds 12 pre-built saved queries

frontend/src/pages/SqlExplorer.tsx

@@ -26,7 +26,7 @@ interface SavedQuery {
 interface SchemaInfo {
   catalog: string;
   schema: string;
-  tables: Array<{ name: string; columns: Array<{ name: string; type: string }> }>;
+  tables: Array<{ name: string; columns: Array<{ name: string; type: string; nullable?: boolean }> }>;
 }
 
 type ChartType = 'none' | 'bar' | 'line' | 'scatter';
@@ -41,8 +41,8 @@ export function SqlExplorerPage() {
   const [yCol, setYCol] = useState(1);
 
   const { data: schema } = useQuery<SchemaInfo>({
-    queryKey: ['trino-schema'],
-    queryFn: () => apiGet<SchemaInfo>('query', '/api/analytics/schema'),
+    queryKey: ['pg-schema'],
+    queryFn: () => apiGet<SchemaInfo>('query', '/api/analytics/pg-schema'),
   });
 
   const { data: savedQueries } = useQuery<SavedQuery[]>({
@@ -51,7 +51,7 @@ export function SqlExplorerPage() {
   });
 
   const executeMutation = useMutation({
-    mutationFn: (sqlText: string) => apiPost<QueryResult>('query', '/api/analytics/query', { sql: sqlText, limit: 1000 }),
+    mutationFn: (sqlText: string) => apiPost<QueryResult>('query', '/api/analytics/pg-query', { sql: sqlText, limit: 1000 }),
     onSuccess: (data) => { setResult(data); setError(null); },
     onError: (err: Error) => { setError(err.message); setResult(null); },
   });
@@ -75,6 +75,11 @@ export function SqlExplorerPage() {
     if (name) saveMutation.mutate({ name, sql_text: sql });
   }, [sql, saveMutation]);
 
+  const handleQuickInsert = useCallback((tableName: string) => {
+    const query = `SELECT * FROM ${tableName} LIMIT 100`;
+    setSql(query);
+  }, []);
+
   // Build chart data from result
   const chartData = result && result.columns.length >= 2
     ? result.rows.map((row) => ({
@@ -84,16 +89,27 @@ export function SqlExplorerPage() {
       }))
     : [];
 
+  // Split saved queries into pre-built (no id-based delete) and user-saved
+  const preBuiltNames = new Set([
+    'Company Overview', 'Recent Recommendations', 'High Confidence Buys',
+    'Trend Windows Summary', 'Market Prices', 'Document Counts by Type',
+    'Global Events', 'Trading Decisions', 'Ingestion Health',
+    'Reserve Pool Ledger', 'Sector Exposure', 'Source Coverage Matrix',
+  ]);
+  const preBuiltQueries = (savedQueries ?? []).filter((sq) => preBuiltNames.has(sq.name));
+  const userQueries = (savedQueries ?? []).filter((sq) => !preBuiltNames.has(sq.name));
+
   return (
     <div className="flex h-[calc(100vh-3rem)] gap-4">
       {/* Schema browser sidebar */}
-      <div className="w-56 shrink-0 overflow-y-auto rounded-lg border border-surface-700 bg-surface-900 p-3">
+      <div className="w-64 shrink-0 overflow-y-auto rounded-lg border border-surface-700 bg-surface-900 p-3">
         <h2 className="mb-2 text-xs font-semibold uppercase tracking-wider text-gray-500">Schema</h2>
         {schema?.tables.map((t) => (
           <div key={t.name} className="mb-2">
             <button
               className="text-xs font-medium text-brand-300 hover:underline"
-              onClick={() => setSql((prev) => prev + ` ${t.name}`)}
+              onClick={() => handleQuickInsert(t.name)}
+              title={`SELECT * FROM ${t.name} LIMIT 100`}
             >
               {t.name}
             </button>
@@ -108,9 +124,35 @@ export function SqlExplorerPage() {
           </div>
         ))}
 
-        {/* Saved queries */}
+        {/* Pre-built queries */}
+        {preBuiltQueries.length > 0 && (
+          <>
+            <h2 className="mb-2 mt-4 text-xs font-semibold uppercase tracking-wider text-gray-500">Pre-built Queries</h2>
+            {preBuiltQueries.map((sq) => (
+              <div key={sq.id} className="mb-1.5">
+                <button
+                  className="block w-full truncate text-left text-xs text-brand-300 hover:text-brand-200"
+                  onClick={() => setSql(sq.sql_text)}
+                  title={sq.sql_text}
+                >
+                  {sq.name}
+                </button>
+                {sq.description && (
+                  <p className="ml-0 truncate text-[10px] text-gray-600" title={sq.description}>
+                    {sq.description}
+                  </p>
+                )}
+              </div>
+            ))}
+          </>
+        )}
+
+        {/* User saved queries */}
         <h2 className="mb-2 mt-4 text-xs font-semibold uppercase tracking-wider text-gray-500">Saved Queries</h2>
-        {(savedQueries ?? []).map((sq) => (
+        {userQueries.length === 0 && (
+          <p className="text-[10px] text-gray-600">No saved queries yet</p>
+        )}
+        {userQueries.map((sq) => (
           <div key={sq.id} className="mb-1 flex items-center justify-between">
             <button
               className="truncate text-xs text-gray-300 hover:text-brand-300"