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/category.js

@@ -0,0 +1,144 @@
+export const GET_TREE_FOR_COMPANY = `
+WITH tree AS (
+    SELECT id, parent_id, sort
+    FROM category
+    WHERE id = @rootId
+
+    UNION ALL
+
+    SELECT c.id, c.parent_id, c.sort
+    FROM category c
+    INNER JOIN tree t ON c.parent_id = t.id
+)
+SELECT
+    t.id,
+    t.parent_id AS parentId,
+    t.sort,
+    ct.name,
+    ct.slug,
+    p.hash AS fullHash,
+    p.thumbw AS thumbW,
+    p.thumbh AS thumbH,
+    p.thumbhash AS thumbHash
+FROM tree t
+LEFT JOIN categorytext ct
+    ON ct.category_id = t.id AND ct.language_id = @languageId
+OUTER APPLY (
+    SELECT TOP 1 cp.pic_id
+    FROM categorypic cp
+    WHERE cp.category_id = t.id
+    ORDER BY cp.sort ASC, cp.id ASC
+) first_pic
+LEFT JOIN pic p ON p.id = first_pic.pic_id
+ORDER BY t.sort, t.id
+`;
+
+import {
+  ARTICLE_LIST_COLUMNS,
+  ARTICLE_LIST_JOINS,
+} from './article.js';
+
+export const GET_CATEGORY_ARTICLES = `
+SELECT
+${ARTICLE_LIST_COLUMNS}
+FROM categoryarticle ca
+INNER JOIN article a ON a.id = ca.article_id
+${ARTICLE_LIST_JOINS}
+WHERE ca.category_id = @categoryId
+ORDER BY a.id
+`;
+
+export const CREATE = `
+INSERT INTO category (parent_id, sort)
+OUTPUT INSERTED.id, INSERTED.parent_id AS parentId, INSERTED.sort
+VALUES (@parentId, @sort)
+`;
+
+export const RESOLVE_CATEGORY_ACCESS = `
+WITH ancestors AS (
+    SELECT id, parent_id
+    FROM category
+    WHERE id = @categoryId
+
+    UNION ALL
+
+    SELECT c.id, c.parent_id
+    FROM category c
+    INNER JOIN ancestors a ON c.id = a.parent_id
+),
+root AS (
+    SELECT TOP 1 id AS rootCategoryId
+    FROM ancestors
+    WHERE parent_id IS NULL
+)
+SELECT
+    p.id AS categoryId,
+    r.rootCategoryId,
+    co.id AS companyId,
+    co.default_language_id AS languageId,
+    co.poscustomergroup_id AS posCustomerGroupId,
+    cu.id AS companyuserId
+FROM category p
+LEFT JOIN root r ON 1 = 1
+LEFT JOIN company co ON co.root_category_id = r.rootCategoryId
+LEFT JOIN companyuser cu
+    ON cu.company_id = co.id AND cu.google_auth_id = @googleAuthId
+WHERE p.id = @categoryId
+`;
+
+export const GET_NEXT_SIBLING_SORT = `
+SELECT ISNULL(MAX(sort), -1) + 1 AS sort
+FROM category
+WHERE parent_id = @parentId
+`;
+
+/** Resolve a child category by its parent id and name (language-sensitive). */
+export const GET_BY_PARENT_AND_NAME = `
+SELECT c.id
+FROM category c
+INNER JOIN categorytext ct
+    ON ct.category_id = c.id AND ct.language_id = @languageId
+WHERE c.parent_id = @parentId
+  AND ct.name = @name
+`;
+
+export const LIST_CHILD_IDS = `
+SELECT id
+FROM category
+WHERE parent_id = @parentId
+`;
+
+export const UPDATE_SORT = `
+UPDATE category
+SET sort = @sort
+WHERE id = @id AND parent_id = @parentId
+`;
+
+/** True if @id is an ancestor of @parentId (would create a cycle when moving). */
+export const IS_ANCESTOR = `
+WITH ancestors AS (
+    SELECT id, parent_id
+    FROM category
+    WHERE id = @parentId
+
+    UNION ALL
+
+    SELECT c.id, c.parent_id
+    FROM category c
+    INNER JOIN ancestors a ON c.id = a.parent_id
+)
+SELECT id
+FROM ancestors
+WHERE id = @id
+`;
+
+export const MOVE = `
+UPDATE category
+SET parent_id = @parentId, sort = @sort
+WHERE id = @id
+`;
+
+export const DELETE = `
+DELETE FROM category
+WHERE id = @id
+`;