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/google_auth.js

@@ -0,0 +1,58 @@
+export const ADD = `
+INSERT INTO google_auth (email)
+OUTPUT INSERTED.id
+VALUES (@email)
+`;
+
+export const GOOGLE_AUTH_COLUMNS = `
+    id,
+    email,
+    picture,
+    picturebinary AS pictureBinary,
+    sub,
+    created_at AS createdAt,
+    selected_company AS selectedCompanyId
+`;
+
+export const GET_BY_EMAIL = `
+SELECT
+${GOOGLE_AUTH_COLUMNS}
+FROM google_auth
+WHERE email = @email
+`;
+
+export const GET_BY_ID = `
+SELECT
+${GOOGLE_AUTH_COLUMNS}
+FROM google_auth
+WHERE id = @googleAuthId
+`;
+
+export const LIST_ALLOWED_EMAILS = `
+SELECT
+    id,
+    email,
+    picture,
+    sub,
+    created_at AS createdAt,
+    selected_company AS selectedCompanyId
+FROM google_auth
+ORDER BY email
+`;
+
+export const REMOVE_ALLOWED_EMAIL = `
+DELETE FROM google_auth
+WHERE email = @email
+`;
+
+export const SET_SELECTED_COMPANY = `
+UPDATE google_auth
+SET selected_company = @companyId
+WHERE id = @googleAuthId
+`;
+
+export const SET_PROFILE = `
+UPDATE google_auth
+SET picture = @picture, picturebinary = @pictureBinary, sub = @sub
+WHERE id = @googleAuthId
+`;