refactor: introduce database provider abstraction, isolate MSSQL layer

- Add provider contract (src/providers/provider.js): query() -> {rows,
  rowsAffected, insertId}, callback transaction(fn), exec() for stored
  procs, initialize/purge lifecycle
- Add MssqlProvider (src/providers/mssql/index.js) wrapping an mssql
  ConnectionPool with normalized results, tx commit/rollback and
  constraint-error classifiers (unique 2627/2601, FK 547)
- LibDashStore constructor now takes a provider instead of a pool
  (breaking); adds LibDashStore.create() async factory; keeps pool as
  transitional bridge until store modules are converted (see TODO.md)
- Move MSSQL-specific code into the provider: src/sql/* ->
  src/providers/mssql/queries/*, src/schema.js ->
  src/providers/mssql/schema.js, purge SQL -> purgeSql.js
- Update scripts, generate-schema-graph.js and package.json exports
  (./providers/mssql, ./providers/provider)
- Add TODO.md with the remaining provider/SQLite migration plan

No SQL or behavioral changes.

src/providers/mssql/purgeSql.js

@@ -0,0 +1,74 @@
+/** Drop order = reverse of schema create dependency order. */
+export const TABLES = [
+  'companyuser',
+  'syncpos',
+  'companysetting',
+  'posterminalcategory',
+  'posterminal',
+  'numbers',
+  'ledger',
+  'articlesupplier',
+  'manifestarticle',
+  'manifest',
+  'price',
+  'articletext',
+  'categorytext',
+  'categorypic',
+  'categoryarticle',
+  'optionslistarticle',
+  'articlelistarticle',
+  'brandpic',
+  'articlepic',
+  'articleean',
+  'articlehistory',
+  'entityhistory',
+  'entityaddress',
+  'entitycontact',
+  'article',
+  'brand',
+  'articlelist',
+  'optionslist',
+  'unittext',
+  'unit',
+  'entity',
+  'customergroup',
+  'google_auth',
+  'company',
+  'pic',
+  'category',
+  'tax',
+  'language',
+  'meta',
+];
+
+export const PROCEDURES = [
+  'getnextnumberwithprefixandpadding',
+  'peeknextnumberwithprefixandpadding',
+];
+
+export const VIEWS = ['v_entity_company_number'];
+
+/** Drop before tables — breaks company ↔ customergroup cycle for purge. */
+export const CONSTRAINTS = [
+  `IF EXISTS (SELECT 1 FROM sys.foreign_keys WHERE name = N'FK_company_poscustomergroup')
+    ALTER TABLE company DROP CONSTRAINT FK_company_poscustomergroup;`,
+];
+
+export function buildPurgeSql() {
+  const drops = [
+    ...PROCEDURES.map(
+      (name) =>
+        `IF OBJECT_ID(N'dbo.${name}', N'P') IS NOT NULL DROP PROCEDURE dbo.[${name}];`,
+    ),
+    ...VIEWS.map(
+      (name) =>
+        `IF OBJECT_ID(N'dbo.${name}', N'V') IS NOT NULL DROP VIEW dbo.[${name}];`,
+    ),
+    ...CONSTRAINTS,
+    ...TABLES.map(
+      (name) =>
+        `IF OBJECT_ID(N'dbo.${name}', N'U') IS NOT NULL DROP TABLE dbo.[${name}];`,
+    ),
+  ];
+  return drops.join('\n');
+}