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/posterminal.js

@@ -0,0 +1,91 @@
+export const LIST_BY_COMPANY = `
+SELECT
+    id,
+    name,
+    token,
+    pairingpin
+FROM posterminal
+WHERE company_id = @companyId
+ORDER BY id
+`;
+
+export const GET_BY_ID = `
+SELECT
+    id,
+    company_id AS companyId,
+    name,
+    token,
+    pairingpin
+FROM posterminal
+WHERE id = @id
+`;
+
+export const CREATE = `
+INSERT INTO posterminal (company_id, name, token, pairingpin)
+OUTPUT INSERTED.id, INSERTED.name, INSERTED.token, INSERTED.pairingpin
+VALUES (@companyId, @name, @token, @pairingpin)
+`;
+
+export const UPDATE = `
+UPDATE posterminal
+SET name = @name, token = @token, pairingpin = @pairingpin
+WHERE id = @id AND company_id = @companyId
+`;
+
+export const DELETE_BY_COMPANY_AND_ID = `
+DELETE FROM posterminal
+WHERE id = @id AND company_id = @companyId
+`;
+
+export const LIST_CATEGORY_IDS = `
+SELECT category_id AS categoryId
+FROM posterminalcategory
+WHERE posterminal_id = @posterminalId
+`;
+
+/** setOn: insert if not exists (single terminal + single category). */
+export const SET_ON = `
+IF NOT EXISTS (
+    SELECT 1 FROM posterminalcategory
+    WHERE posterminal_id = @posterminalId AND category_id = @categoryId
+)
+INSERT INTO posterminalcategory (posterminal_id, category_id)
+VALUES (@posterminalId, @categoryId)
+`;
+
+/** setOff: remove a single category from a single terminal. */
+export const SET_OFF = `
+DELETE FROM posterminalcategory
+WHERE posterminal_id = @posterminalId AND category_id = @categoryId
+`;
+
+/** setOnAll: enable ALL company categories on one terminal. */
+export const SET_ON_ALL = `
+;WITH cats AS (
+    SELECT id FROM category WHERE id = @rootCategoryId
+    UNION ALL
+    SELECT c.id FROM category c INNER JOIN cats ON c.parent_id = cats.id
+)
+INSERT INTO posterminalcategory (posterminal_id, category_id)
+SELECT @posterminalId, cats.id
+FROM cats
+WHERE NOT EXISTS (
+    SELECT 1 FROM posterminalcategory pc
+    WHERE pc.posterminal_id = @posterminalId AND pc.category_id = cats.id
+)
+`;
+
+/** setOffAll: remove ALL category links from one terminal. */
+export const SET_OFF_ALL = `
+DELETE FROM posterminalcategory WHERE posterminal_id = @posterminalId
+`;
+
+/** For category tree: map category -> posterminal ids (company-scoped). */
+export const POSTERMINAL_IDS_BY_COMPANY = `
+SELECT
+    pc.category_id AS categoryId,
+    pc.posterminal_id AS posterminalId
+FROM posterminalcategory pc
+INNER JOIN posterminal pt ON pt.id = pc.posterminal_id
+WHERE pt.company_id = @companyId
+`;