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/articleDetail.js

@@ -0,0 +1,124 @@
+export const ARTICLE_DETAIL_COLUMNS = `
+    a.id,
+    a.sku,
+    at.name,
+    at.slug,
+    a.unitamount,
+    a.weight,
+    u.si AS unitSi,
+    u.basesi AS unitBasesi,
+    u.basesifactor AS unitBasesifactor,
+    ut.symbol AS unitSymbol,
+    ut.name AS unitName,
+    b.name AS brandName,
+    brand_pic.thumbw AS brandThumbW,
+    brand_pic.thumbh AS brandThumbH,
+    brand_pic.thumbhash AS brandThumbHash,
+    CASE
+        WHEN a.optionslist_id IS NOT NULL THEN (
+            SELECT COUNT(*)
+            FROM optionslistarticle ola
+            WHERE ola.optionslist_id = a.optionslist_id
+        )
+        ELSE 0
+    END AS optionsCount,
+    CASE
+        WHEN a.articlelist_id IS NOT NULL THEN (
+            SELECT COUNT(*)
+            FROM articlelistarticle ala
+            WHERE ala.articlelist_id = a.articlelist_id
+        )
+        ELSE 0
+    END AS articleListCount,
+    (
+        SELECT COUNT(*)
+        FROM article child
+        WHERE child.fatherarticle_id = a.id
+    ) AS childCount,
+    a.fatherarticle_id AS fatherArticleId,
+    father.sku AS fatherSku
+`;
+
+export const GET_DETAIL_BY_SKU = `
+SELECT
+${ARTICLE_DETAIL_COLUMNS}
+FROM article a
+LEFT JOIN articletext at
+    ON at.article_id = a.id AND at.language_id = @languageId
+LEFT JOIN unit u ON u.id = a.unit_id
+LEFT JOIN unittext ut
+    ON ut.unit_id = u.id AND ut.language_id = @languageId
+LEFT JOIN brand b ON b.id = a.brand_id
+OUTER APPLY (
+    SELECT TOP 1 bp.pic_id
+    FROM brandpic bp
+    WHERE bp.brand_id = b.id
+    ORDER BY bp.sort ASC, bp.id ASC
+) first_brand_pic
+LEFT JOIN pic brand_pic ON brand_pic.id = first_brand_pic.pic_id
+LEFT JOIN article father ON father.id = a.fatherarticle_id
+WHERE a.sku = @sku
+`;
+
+export const GET_DETAIL_BY_ARTICLE_ID = `
+SELECT
+${ARTICLE_DETAIL_COLUMNS}
+FROM article a
+LEFT JOIN articletext at
+    ON at.article_id = a.id AND at.language_id = @languageId
+LEFT JOIN unit u ON u.id = a.unit_id
+LEFT JOIN unittext ut
+    ON ut.unit_id = u.id AND ut.language_id = @languageId
+LEFT JOIN brand b ON b.id = a.brand_id
+OUTER APPLY (
+    SELECT TOP 1 bp.pic_id
+    FROM brandpic bp
+    WHERE bp.brand_id = b.id
+    ORDER BY bp.sort ASC, bp.id ASC
+) first_brand_pic
+LEFT JOIN pic brand_pic ON brand_pic.id = first_brand_pic.pic_id
+LEFT JOIN article father ON father.id = a.fatherarticle_id
+WHERE a.id = @articleId
+`;
+
+export const GET_CATEGORY_IDS_BY_ARTICLE_ID = `
+SELECT ca.category_id AS categoryId
+FROM categoryarticle ca
+WHERE ca.article_id = @articleId
+ORDER BY ca.category_id
+`;
+
+export const GET_PHOTOS_BY_ARTICLE_ID = `
+SELECT
+    ap.sort,
+    p.hash AS fullHash,
+    p.w AS fullW,
+    p.h AS fullH,
+    p.size AS fullSize,
+    p.thumbw AS thumbW,
+    p.thumbh AS thumbH,
+    p.thumbhash AS thumbHash
+FROM articlepic ap
+INNER JOIN pic p ON p.id = ap.pic_id
+WHERE ap.article_id = @articleId
+ORDER BY ap.sort ASC, ap.id ASC
+`;
+
+export const GET_EANS_BY_ARTICLE_ID = `
+SELECT ae.ean
+FROM articleean ae
+WHERE ae.article_id = @articleId
+ORDER BY ae.ean
+`;
+
+export const GET_PRICES_BY_ARTICLE_ID = `
+SELECT
+    pr.minmultiplier AS minmultiplier,
+    pr.net,
+    t.rate AS taxRate,
+    pr.customergroup_id AS customergroupId
+FROM price pr
+INNER JOIN tax t ON t.id = pr.tax_id
+WHERE pr.article_id = @articleId
+ORDER BY pr.customergroup_id, pr.minmultiplier
+`;