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/unit/delete.js

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

src/store/unit/ensureDefault.js

@@ -1,8 +1,8 @@
-import { CREATE, GET_BY_SI } from '../../sql/unit.js';
+import { CREATE, GET_BY_SI } from '../../providers/mssql/queries/unit.js';
 import {
   CREATE as CREATE_UNITTEXT,
   GET_BY_UNIT_AND_LANGUAGE,
-} from '../../sql/unittext.js';
+} from '../../providers/mssql/queries/unittext.js';
 import request from '../request.js';
 
 const DEFAULT_SI = 'pc';

src/store/unit/list.js

@@ -1,4 +1,4 @@
-import { LIST_BY_COMPANY } from '../../sql/unit.js';
+import { LIST_BY_COMPANY } from '../../providers/mssql/queries/unit.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 import shapeUnit from './shapeUnit.js';
 

src/store/unit/upsert.js

@@ -3,12 +3,12 @@ import {
   GET_BY_SI,
   GET_FULL_BY_SI,
   UPDATE_BY_SI,
-} from '../../sql/unit.js';
+} from '../../providers/mssql/queries/unit.js';
 import {
   CREATE as CREATE_UNITTEXT,
   GET_BY_UNIT_AND_LANGUAGE,
   UPDATE_BY_UNIT_AND_LANGUAGE,
-} from '../../sql/unittext.js';
+} from '../../providers/mssql/queries/unittext.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 import request from '../request.js';
 import shapeUnit from './shapeUnit.js';