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/articleDelete.js

@@ -0,0 +1,34 @@
+export const GET_CHILD_COUNT = `
+SELECT COUNT(*) AS count
+FROM article
+WHERE fatherarticle_id = @articleId
+`;
+
+export const GET_LIST_IDS = `
+SELECT articlelist_id AS articlelistId, optionslist_id AS optionslistId
+FROM article
+WHERE id = @articleId
+`;
+
+
+export const DELETE_PICS_BY_ARTICLE = `
+DELETE p
+FROM pic p
+INNER JOIN articlepic ap ON ap.pic_id = p.id
+WHERE ap.article_id = @articleId
+`;
+
+export const DELETE_BY_ID = `
+DELETE FROM article
+WHERE id = @articleId
+`;
+
+export const DELETE_ARTICLELIST = `
+DELETE FROM articlelist
+WHERE id = @id
+`;
+
+export const DELETE_OPTIONSLIST = `
+DELETE FROM optionslist
+WHERE id = @id
+`;