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/supplier/create.js

@@ -1,5 +1,5 @@
-import { CREATE } from '../../sql/entity.js';
-import { INSERT as INSERT_HISTORY } from '../../sql/entityhistory.js';
+import { CREATE } from '../../providers/mssql/queries/entity.js';
+import { INSERT as INSERT_HISTORY } from '../../providers/mssql/queries/entityhistory.js';
 import request from '../request.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 import ensureLieferantenGroup from './ensureLieferantenGroup.js';

src/store/supplier/delete.js

@@ -1,4 +1,4 @@
-import { DELETE_BY_ID, SUPPLIER_GROUP_NAME } from '../../sql/entity.js';
+import { DELETE_BY_ID, SUPPLIER_GROUP_NAME } from '../../providers/mssql/queries/entity.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 
 export default function del(pool) {

src/store/supplier/ensureLieferantenGroup.js

@@ -2,8 +2,8 @@ import {
   CREATE,
   GET_BY_COMPANY_AND_NAME,
   GET_NEXT_SORT,
-} from '../../sql/customergroup.js';
-import { SUPPLIER_GROUP_NAME } from '../../sql/entity.js';
+} from '../../providers/mssql/queries/customergroup.js';
+import { SUPPLIER_GROUP_NAME } from '../../providers/mssql/queries/entity.js';
 
 /** Ensure company has customergroup "Lieferanten"; return its id. */
 export default async function ensureLieferantenGroup(pool, companyId, tx) {

src/store/supplier/list.js

@@ -1,4 +1,4 @@
-import { LIST_SUPPLIERS_BY_COMPANY, SUPPLIER_GROUP_NAME } from '../../sql/entity.js';
+import { LIST_SUPPLIERS_BY_COMPANY, SUPPLIER_GROUP_NAME } from '../../providers/mssql/queries/entity.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 
 export default function list(pool) {

src/store/supplier/update.js

@@ -3,8 +3,8 @@ import {
   GET_SUPPLIER_BY_ID,
   SUPPLIER_GROUP_NAME,
   UPDATE_NAME,
-} from '../../sql/entity.js';
-import { INSERT as INSERT_HISTORY } from '../../sql/entityhistory.js';
+} from '../../providers/mssql/queries/entity.js';
+import { INSERT as INSERT_HISTORY } from '../../providers/mssql/queries/entityhistory.js';
 import request from '../request.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';