Fix filing taxonomy schema mismatch by adding explicit column verification

The filing_taxonomy_snapshot table was missing parser_engine and related columns
on databases created before the taxonomy surface sidecar migration. This caused
filing sync workflows to fail with 'table has no column named parser_engine'.

Changes:
- Add TAXONOMY_SNAPSHOT_REQUIRED_COLUMNS constant for required columns
- Add verifyCriticalSchema() to fail fast at startup if schema is incompatible
- Reorder ensureTaxonomySnapshotCompat to check table existence before column ops
- Add explicit column verification after ALTER TABLE attempts
- Add regression tests for missing column detection

Fixes #2

lib/server/db/index.test.ts

@@ -199,4 +199,35 @@ describe('sqlite schema compatibility bootstrap', () => {
 
     client.close();
   });
+
+  it('throws on missing parser_engine column when verifyCriticalSchema is called', () => {
+    const client = new Database(':memory:');
+    client.exec('PRAGMA foreign_keys = ON;');
+
+    applyMigration(client, '0000_cold_silver_centurion.sql');
+    applyMigration(client, '0005_financial_taxonomy_v3.sql');
+
+    expect(__dbInternals.hasTable(client, 'filing_taxonomy_snapshot')).toBe(true);
+    expect(__dbInternals.hasColumn(client, 'filing_taxonomy_snapshot', 'parser_engine')).toBe(false);
+
+    expect(() => __dbInternals.verifyCriticalSchema(client)).toThrow(
+      /filing_taxonomy_snapshot is missing columns: parser_engine/
+    );
+
+    client.close();
+  });
+
+  it('verifyCriticalSchema passes when all required columns exist', () => {
+    const client = new Database(':memory:');
+    client.exec('PRAGMA foreign_keys = ON;');
+
+    applyMigration(client, '0000_cold_silver_centurion.sql');
+    applyMigration(client, '0005_financial_taxonomy_v3.sql');
+
+    __dbInternals.ensureLocalSqliteSchema(client);
+
+    expect(() => __dbInternals.verifyCriticalSchema(client)).not.toThrow();
+
+    client.close();
+  });
 });