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/quote/create.js

@@ -1,4 +1,4 @@
-import { CREATE_MANIFEST, QUOTE_TYPE } from '../../sql/manifestDoc.js';
+import { CREATE_MANIFEST, QUOTE_TYPE } from '../../providers/mssql/queries/manifestDoc.js';
 import request from '../request.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 import taxEnsure from '../tax/ensure.js';

src/store/quote/delete.js

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

src/store/quote/getById.js

@@ -1,4 +1,4 @@
-import { QUOTE_TYPE } from '../../sql/manifestDoc.js';
+import { QUOTE_TYPE } from '../../providers/mssql/queries/manifestDoc.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 import { loadDoc } from '../manifestDoc/helpers.js';
 

src/store/quote/list.js

@@ -1,4 +1,4 @@
-import { LIST_MANIFESTS, QUOTE_TYPE } from '../../sql/manifestDoc.js';
+import { LIST_MANIFESTS, QUOTE_TYPE } from '../../providers/mssql/queries/manifestDoc.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 import { shapeSummary } from '../manifestDoc/helpers.js';
 

src/store/quote/update.js

@@ -3,7 +3,7 @@ import {
   GET_MANIFEST_HEADER,
   QUOTE_TYPE,
   UPDATE_MANIFEST,
-} from '../../sql/manifestDoc.js';
+} from '../../providers/mssql/queries/manifestDoc.js';
 import request from '../request.js';
 import resolveSelectedCompanyAccess from '../companyuser/resolveSelectedCompanyAccess.js';
 import taxEnsure from '../tax/ensure.js';