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/entitycontact.js

@@ -0,0 +1,56 @@
+export const CONTACT_COLUMNS = `
+    id,
+    entity_id AS entityId,
+    contacttype AS contactType,
+    contact,
+    sort
+`;
+
+export const LIST_BY_ENTITY = `
+SELECT
+${CONTACT_COLUMNS}
+FROM entitycontact
+WHERE entity_id = @entityId
+ORDER BY sort ASC, id ASC
+`;
+
+export const GET_BY_ENTITY_AND_ID = `
+SELECT
+${CONTACT_COLUMNS}
+FROM entitycontact
+WHERE id = @id AND entity_id = @entityId
+`;
+
+export const GET_NEXT_SORT = `
+SELECT ISNULL(MAX(sort), -1) + 1 AS sort
+FROM entitycontact
+WHERE entity_id = @entityId
+`;
+
+export const CREATE = `
+INSERT INTO entitycontact (entity_id, contacttype, contact, sort)
+OUTPUT
+    INSERTED.id,
+    INSERTED.entity_id AS entityId,
+    INSERTED.contacttype AS contactType,
+    INSERTED.contact,
+    INSERTED.sort
+VALUES (@entityId, @contactType, @contact, @sort)
+`;
+
+export const UPDATE = `
+UPDATE entitycontact
+SET contacttype = @contactType, contact = @contact, sort = @sort
+OUTPUT
+    INSERTED.id,
+    INSERTED.entity_id AS entityId,
+    INSERTED.contacttype AS contactType,
+    INSERTED.contact,
+    INSERTED.sort
+WHERE id = @id AND entity_id = @entityId
+`;
+
+export const DELETE_BY_ENTITY_AND_ID = `
+DELETE FROM entitycontact
+WHERE id = @id AND entity_id = @entityId
+`;