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/company.js

@@ -0,0 +1,69 @@
+export const GET_BY_NAME = `
+SELECT id, name, root_category_id, default_language_id, taxrate, reducedtaxrate, taxfreerate
+FROM company
+WHERE name = @name
+`;
+
+export const GET_BY_ID = `
+SELECT id, name, root_category_id, default_language_id, taxrate, reducedtaxrate, taxfreerate
+FROM company
+WHERE id = @id
+`;
+
+export const GET_COMPANY_ROOT_CATEGORY = `
+SELECT root_category_id AS rootCategoryId FROM company WHERE id = @companyId
+`;
+
+export const CREATE = `
+INSERT INTO company (
+    name,
+    root_category_id,
+    default_language_id,
+    taxrate,
+    reducedtaxrate,
+    taxfreerate,
+    poscustomergroup_id
+)
+OUTPUT INSERTED.id
+VALUES (
+    @name,
+    @rootCategoryId,
+    @languageId,
+    @taxrate,
+    @reducedtaxrate,
+    @taxfreerate,
+    @posCustomerGroupId
+)
+`;
+
+export const UPDATE = `
+UPDATE company
+SET
+    name = @name,
+    default_language_id = @languageId,
+    taxrate = @taxrate,
+    reducedtaxrate = @reducedtaxrate
+WHERE id = @id
+`;
+
+export const LIST = `
+SELECT
+    c.id,
+    c.name,
+    c.root_category_id AS rootCategoryId,
+    c.default_language_id AS languageId,
+    tr.rate AS taxrate,
+    rtr.rate AS reducedtaxrate,
+    tfr.rate AS taxfreerate
+FROM company c
+INNER JOIN companyuser cu ON cu.company_id = c.id
+LEFT JOIN tax tr ON tr.id = c.taxrate
+LEFT JOIN tax rtr ON rtr.id = c.reducedtaxrate
+LEFT JOIN tax tfr ON tfr.id = c.taxfreerate
+WHERE cu.google_auth_id = @googleAuthId
+`;
+
+export const DELETE = `
+DELETE FROM company
+WHERE id = @companyId
+`;