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/unit.js

@@ -0,0 +1,50 @@
+export const UNIT_COLUMNS = `
+    u.si,
+    u.basesi,
+    u.basesifactor,
+    ut.symbol,
+    ut.name
+`;
+
+export const GET_BY_SI = `
+SELECT id
+FROM unit
+WHERE company_id = @companyId AND si = @si
+`;
+
+export const GET_FULL_BY_SI = `
+SELECT
+    u.id,
+${UNIT_COLUMNS}
+FROM unit u
+LEFT JOIN unittext ut
+    ON ut.unit_id = u.id AND ut.language_id = @languageId
+WHERE u.company_id = @companyId AND u.si = @si
+`;
+
+export const LIST_BY_COMPANY = `
+SELECT
+${UNIT_COLUMNS}
+FROM unit u
+LEFT JOIN unittext ut
+    ON ut.unit_id = u.id AND ut.language_id = @languageId
+WHERE u.company_id = @companyId
+ORDER BY ut.symbol, ut.name, u.si
+`;
+
+export const CREATE = `
+INSERT INTO unit (company_id, si, basesi, basesifactor)
+OUTPUT INSERTED.id
+VALUES (@companyId, @si, @basesi, @basesifactor)
+`;
+
+export const UPDATE_BY_SI = `
+UPDATE unit
+SET basesi = @basesi, basesifactor = @basesifactor
+WHERE company_id = @companyId AND si = @si
+`;
+
+export const DELETE_BY_COMPANY_AND_SI = `
+DELETE FROM unit
+WHERE company_id = @companyId AND si = @si
+`;