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/store/article/updateBySku.js

@@ -2,11 +2,11 @@ import {
   UPDATE_UNIT,
   UPDATE_UNITAMOUNT,
   UPDATE_UNIT_AND_AMOUNT,
-} from '../../sql/article.js';
-import { INSERT as INSERT_HISTORY } from '../../sql/articlehistory.js';
-import { UPDATE_BY_ARTICLE_AND_LANGUAGE } from '../../sql/articletext.js';
-import { GET_DETAIL_BY_ARTICLE_ID } from '../../sql/articleDetail.js';
-import { GET_FULL_BY_SI } from '../../sql/unit.js';
+} from '../../providers/mssql/queries/article.js';
+import { INSERT as INSERT_HISTORY } from '../../providers/mssql/queries/articlehistory.js';
+import { UPDATE_BY_ARTICLE_AND_LANGUAGE } from '../../providers/mssql/queries/articletext.js';
+import { GET_DETAIL_BY_ARTICLE_ID } from '../../providers/mssql/queries/articleDetail.js';
+import { GET_FULL_BY_SI } from '../../providers/mssql/queries/unit.js';
 import request from '../request.js';
 import queryBySku from './queryBySku.js';
 import resolveArticleBySku from './resolveArticleBySku.js';