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/providers/mssql/queries/articlesupplier.js

@@ -0,0 +1,174 @@
+export const LIST_BY_ARTICLE_ID = `
+SELECT
+    asup.id,
+    asup.article_id AS articleId,
+    asup.entity_id AS entityId,
+    e.name AS supplierName,
+    asup.sku,
+    asup.pu,
+    asup.stock,
+    asup.sort,
+    asup.price_id AS priceId,
+    p.net AS priceNet,
+    p.tax_id AS priceTaxId,
+    t.rate AS priceTaxRate
+FROM articlesupplier asup
+INNER JOIN entity e ON e.id = asup.entity_id
+INNER JOIN customergroup cg ON cg.id = e.customergroup_id
+LEFT JOIN price p ON p.id = asup.price_id
+LEFT JOIN tax t ON t.id = p.tax_id
+WHERE asup.article_id = @articleId
+  AND cg.company_id = @companyId
+ORDER BY asup.sort ASC, asup.id ASC;
+`;
+
+export const GET_BY_ID_AND_ARTICLE_ID = `
+SELECT
+    asup.id,
+    asup.article_id AS articleId,
+    asup.entity_id AS entityId,
+    e.name AS supplierName,
+    asup.sku,
+    asup.pu,
+    asup.stock,
+    asup.sort,
+    asup.price_id AS priceId,
+    p.net AS priceNet,
+    p.tax_id AS priceTaxId,
+    t.rate AS priceTaxRate
+FROM articlesupplier asup
+INNER JOIN entity e ON e.id = asup.entity_id
+INNER JOIN customergroup cg ON cg.id = e.customergroup_id
+LEFT JOIN price p ON p.id = asup.price_id
+LEFT JOIN tax t ON t.id = p.tax_id
+WHERE asup.id = @id
+  AND asup.article_id = @articleId
+  AND cg.company_id = @companyId;
+`;
+
+export const CHECK_EXISTS = `
+SELECT TOP 1 id
+FROM articlesupplier
+WHERE article_id = @articleId AND entity_id = @entityId;
+`;
+
+export const GET_NEXT_SORT = `
+SELECT ISNULL(MAX(sort), -1) + 1 AS sort
+FROM articlesupplier
+WHERE article_id = @articleId;
+`;
+
+export const CREATE = `
+INSERT INTO articlesupplier (article_id, entity_id, price_id, sku, stock, pu, sort)
+OUTPUT INSERTED.id
+VALUES (@articleId, @entityId, @priceId, @sku, @stock, @pu, @sort);
+`;
+
+export const UPDATE = `
+UPDATE articlesupplier
+SET
+    sku = @sku,
+    stock = @stock,
+    pu = @pu,
+    price_id = @priceId,
+    sort = @sort
+WHERE id = @id AND article_id = @articleId;
+`;
+
+export const DELETE_BY_ID_AND_ARTICLE_ID = `
+DELETE FROM articlesupplier
+WHERE id = @id AND article_id = @articleId;
+`;
+
+export const LIST_IDS_BY_ARTICLE = `
+SELECT id
+FROM articlesupplier
+WHERE article_id = @articleId
+ORDER BY sort ASC, id ASC;
+`;
+
+export const SET_TEMP_SORTS = `
+UPDATE articlesupplier
+SET sort = -id
+WHERE article_id = @articleId;
+`;
+
+export const UPDATE_SORT = `
+UPDATE articlesupplier
+SET sort = @sort
+WHERE id = @id AND article_id = @articleId;
+`;
+
+export const UPDATE_PRICE = `
+UPDATE price
+SET
+    net = @net,
+    tax_id = @taxId
+WHERE id = @id AND article_id = @articleId;
+`;
+
+export const DELETE_PRICE = `
+DELETE FROM price
+WHERE id = @id AND article_id = @articleId;
+`;
+
+export const GET_COMPANY_DEFAULT_TAX = `
+SELECT taxrate
+FROM company
+WHERE id = @companyId;
+`;
+
+export const LIST_BY_ENTITY_AND_SKUS = `
+SELECT
+    asup.id,
+    asup.article_id AS articleId,
+    asup.entity_id AS entityId,
+    e.name AS supplierName,
+    asup.sku,
+    asup.pu,
+    asup.stock,
+    asup.sort,
+    asup.price_id AS priceId,
+    p.net AS priceNet,
+    p.tax_id AS priceTaxId,
+    t.rate AS priceTaxRate,
+    a.id AS a_id,
+    a.sku AS a_sku,
+    at.name AS a_name,
+    at.slug AS a_slug,
+    (
+        SELECT
+            ap.sort,
+            p.hash,
+            p.thumbhash,
+            p.thumbw,
+            p.thumbh
+        FROM articlepic ap
+        INNER JOIN pic p ON p.id = ap.pic_id
+        WHERE ap.article_id = a.id
+        ORDER BY ap.sort ASC, ap.id ASC
+        FOR JSON PATH
+    ) AS photosJson,
+    (
+        SELECT ae.ean
+        FROM articleean ae
+        WHERE ae.article_id = a.id
+        ORDER BY ae.ean
+        FOR JSON PATH
+    ) AS eansJson
+FROM articlesupplier asup
+INNER JOIN entity e ON e.id = asup.entity_id
+INNER JOIN customergroup cg ON cg.id = e.customergroup_id
+INNER JOIN article a ON a.id = asup.article_id
+LEFT JOIN articletext at ON at.article_id = a.id AND at.language_id = @languageId
+LEFT JOIN price p ON p.id = asup.price_id
+LEFT JOIN tax t ON t.id = p.tax_id
+WHERE asup.entity_id = @entityId
+  AND cg.company_id = @companyId
+  AND (
+    @skusJson IS NULL
+    OR a.sku IN (SELECT value FROM OPENJSON(@skusJson))
+    OR asup.sku IN (SELECT value FROM OPENJSON(@skusJson))
+  )
+ORDER BY asup.sort ASC, asup.id ASC;
+`;