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/test/mocks/handlers.ts

@@ -95,6 +95,26 @@ export const handlers = [
     return HttpResponse.json({ id: 'w1', name: body.name, description: body.description ?? null, active: true }, { status: 201 });
   }),
 
+  // Analytics: PostgreSQL schema + query
+  http.get('/api/analytics/pg-schema', () => HttpResponse.json({
+    catalog: 'postgresql', schema: 'public',
+    tables: [
+      { name: 'companies', columns: [{ name: 'id', type: 'uuid', nullable: false }, { name: 'ticker', type: 'character varying', nullable: false }, { name: 'legal_name', type: 'text', nullable: false }] },
+      { name: 'recommendations', columns: [{ name: 'id', type: 'uuid', nullable: false }, { name: 'ticker', type: 'character varying', nullable: false }, { name: 'action', type: 'character varying', nullable: false }] },
+    ],
+  })),
+  http.post('/api/analytics/pg-query', () => HttpResponse.json({
+    columns: [{ name: 'test', type: 'text' }], rows: [['1']], row_count: 1, elapsed_ms: 5,
+  })),
+  http.get('/api/analytics/saved-queries', () => HttpResponse.json([
+    { id: 'sq1', name: 'Company Overview', description: 'All tracked companies with sector and status', sql_text: 'SELECT ticker, legal_name, sector, active, created_at FROM companies ORDER BY ticker', created_at: '2026-04-10T12:00:00Z' },
+  ])),
+  http.post('/api/analytics/saved-queries', async ({ request }) => {
+    const body = await request.json() as Record<string, string>;
+    return HttpResponse.json({ id: 'sq-new', name: body.name, description: '', sql_text: body.sql_text, created_at: '2026-04-10T12:00:00Z' }, { status: 201 });
+  }),
+  http.delete('/api/analytics/saved-queries/:id', () => HttpResponse.json({ status: 'deleted' })),
+
   // Health
   http.get('/api/health', () => HttpResponse.json({ status: 'ok' })),