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/customergroup.js

@@ -0,0 +1,66 @@
+export const CREATE = `
+INSERT INTO customergroup (name, company_id, sort)
+OUTPUT INSERTED.id, INSERTED.name, INSERTED.sort
+VALUES (@name, @companyId, @sort)
+`;
+
+export const CREATE_WITHOUT_COMPANY = `
+INSERT INTO customergroup (name, sort)
+OUTPUT INSERTED.id, INSERTED.name, INSERTED.sort
+VALUES (@name, @sort)
+`;
+
+export const SET_COMPANY = `
+UPDATE customergroup
+SET company_id = @companyId
+WHERE id = @id
+`;
+
+export const GET_BY_COMPANY_AND_NAME = `
+SELECT id, name, sort
+FROM customergroup
+WHERE company_id = @companyId AND name = @name
+`;
+
+export const LIST_BY_COMPANY = `
+SELECT id, name, sort
+FROM customergroup
+WHERE company_id = @companyId
+ORDER BY sort ASC, name ASC, id ASC
+`;
+
+export const GET_BY_COMPANY_AND_ID = `
+SELECT id, name, sort
+FROM customergroup
+WHERE id = @id AND company_id = @companyId
+`;
+
+export const GET_NEXT_SORT = `
+SELECT ISNULL(MAX(sort), -1) + 1 AS sort
+FROM customergroup
+WHERE company_id = @companyId
+`;
+
+export const LIST_IDS_BY_COMPANY = `
+SELECT id
+FROM customergroup
+WHERE company_id = @companyId
+`;
+
+export const UPDATE = `
+UPDATE customergroup
+SET name = @name
+OUTPUT INSERTED.id, INSERTED.name, INSERTED.sort
+WHERE id = @id AND company_id = @companyId
+`;
+
+export const UPDATE_SORT = `
+UPDATE customergroup
+SET sort = @sort
+WHERE id = @id AND company_id = @companyId
+`;
+
+export const DELETE_BY_COMPANY_AND_ID = `
+DELETE FROM customergroup
+WHERE id = @id AND company_id = @companyId
+`;