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.

generate-schema-graph.js

@@ -4,7 +4,7 @@ import { fileURLToPath } from 'node:url';
 import { Graphviz } from '@hpcc-js/wasm-graphviz';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
-const schemaPath = join(__dirname, 'src', 'schema.js');
+const schemaPath = join(__dirname, 'src', 'providers', 'mssql', 'schema.js');
 const outputPath = join(__dirname, 'schema-graph.svg');
 const dotOutputPath = join(__dirname, 'schema-graph.dot');