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/index.js

@@ -123,7 +123,24 @@ import manifestattachmentGetById from './store/manifestattachment/getById.js';
 import manifestattachmentOrder from './store/manifestattachment/order.js';
 
 export default class LibDashStore {
-  constructor(pool) {
+  /**
+   * @param {import('./providers/provider.js').DatabaseProvider} provider
+   */
+  constructor(provider) {
+    if (
+      !provider ||
+      typeof provider.query !== 'function' ||
+      typeof provider.transaction !== 'function'
+    ) {
+      throw new TypeError(
+        'LibDashStore requires a database provider (query/transaction/initialize/purge).',
+      );
+    }
+
+    this.provider = provider;
+    // Transitional: store modules still use the raw mssql pool.
+    // They are converted to provider.query() area by area (see TODO.md).
+    const pool = provider.pool;
     this.pool = pool;
 
     this.companyuser = {
@@ -317,11 +334,16 @@ export default class LibDashStore {
       order: manifestattachmentOrder(pool),
     };
 
-    this.purge = storePurge(pool);
-    this.initialize = storeInitialize(pool);
+    this.purge = storePurge(provider);
+    this.initialize = storeInitialize(provider);
 
     return this.initialize().then(() => this);
   }
+
+  /** Recommended async factory (constructor returning a promise is confusing). */
+  static async create(provider) {
+    return new LibDashStore(provider);
+  }
 }
 
-export { SCHEMA_SCRIPTS } from './schema.js';
+export { SCHEMA_SCRIPTS } from './providers/mssql/schema.js';