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/numbers.js

@@ -0,0 +1,46 @@
+export const ENSURE_TYPE = `
+IF NOT EXISTS (
+    SELECT 1
+    FROM numbers
+    WHERE company_id = @companyId AND [type] = @type
+)
+BEGIN
+    INSERT INTO numbers (company_id, [type])
+    VALUES (@companyId, @type);
+END
+`;
+
+export const LIST_BY_COMPANY = `
+SELECT
+    id,
+    [type],
+    prefix,
+    padding,
+    [next] AS [next]
+FROM numbers
+WHERE company_id = @companyId
+ORDER BY [type]
+`;
+
+export const GET_BY_COMPANY_AND_ID = `
+SELECT
+    id,
+    [type],
+    prefix,
+    padding,
+    [next] AS [next]
+FROM numbers
+WHERE id = @id
+  AND company_id = @companyId
+`;
+
+export const UPDATE_BY_COMPANY_AND_ID = `
+UPDATE numbers
+SET
+    prefix = @prefix,
+    padding = @padding,
+    [next] = @next
+OUTPUT INSERTED.id, INSERTED.[type], INSERTED.prefix, INSERTED.padding, INSERTED.[next]
+WHERE id = @id
+  AND company_id = @companyId
+`;