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/delivery.js

@@ -0,0 +1,117 @@
+export const DELIVERY_TYPE = 'delivery';
+
+export const LIST_DELIVERIES = `
+SELECT
+    m.id,
+    m.[number],
+    m.[type],
+    m.entity_id AS supplierId,
+    m.external_number AS externalNumber,
+    m.[date]
+FROM manifest m
+WHERE m.company_id = @companyId
+  AND m.[type] = @type
+ORDER BY m.[date] DESC, m.id DESC
+`;
+
+export const GET_DELIVERY_HEADER = `
+SELECT
+    m.id,
+    m.[number],
+    m.[type],
+    m.entity_id AS supplierId,
+    m.external_number AS externalNumber,
+    m.[date]
+FROM manifest m
+WHERE m.id = @id
+  AND m.company_id = @companyId
+  AND m.[type] = @type
+`;
+
+export const GET_DELIVERY_LINES = `
+SELECT
+    ma.id,
+    ma.sku,
+    ma.name,
+    ma.pu,
+    ma.amount,
+    ma.net,
+    ma.rebate,
+    ma.taxrate,
+    ma.comment
+FROM manifestarticle ma
+WHERE ma.manifest_id = @manifestId
+ORDER BY ma.id
+`;
+
+export const CREATE_MANIFEST = `
+INSERT INTO manifest (company_id, [number], [type], entity_id, external_number, [date])
+OUTPUT INSERTED.id
+VALUES (@companyId, @number, @type, @entityId, @externalNumber, @date)
+`;
+
+export const UPDATE_MANIFEST = `
+UPDATE manifest
+SET
+    [number] = @number,
+    entity_id = @entityId,
+    external_number = @externalNumber,
+    [date] = @date
+WHERE id = @id
+  AND company_id = @companyId
+  AND [type] = @type
+`;
+
+export const DELETE_MANIFEST = `
+DELETE FROM manifest
+WHERE id = @id
+  AND company_id = @companyId
+  AND [type] = @type
+`;
+
+export const DELETE_LINES_BY_MANIFEST = `
+DELETE FROM manifestarticle
+WHERE manifest_id = @manifestId
+`;
+
+export const GET_NEXT_ATTACHMENT_SORT = `
+SELECT ISNULL(MAX(ma.sort), -1) + 1 AS sort
+FROM manifestattachment ma
+WHERE ma.manifest_id = @manifestId
+`;
+
+export const CREATE_FILE = `
+INSERT INTO [file] (hash, mimetype, name, size, created, modified, content, thumbavif, thumbx, thumby, meta)
+OUTPUT INSERTED.id
+VALUES (@hash, @mimetype, @name, @size, @created, @modified, @content, CONVERT(VARBINARY(MAX), @thumbavif), @thumbx, @thumby, @meta)
+`;
+
+export const CREATE_ATTACHMENT = `
+INSERT INTO manifestattachment (manifest_id, file_id, sort)
+OUTPUT INSERTED.id
+VALUES (@manifestId, @fileId, @sort)
+`;
+
+export const LIST_DELIVERY_FILES = `
+SELECT
+    ma.id,
+    ma.manifest_id AS manifestId,
+    ma.file_id AS fileId,
+    ma.sort,
+    f.hash,
+    f.mimetype,
+    f.name,
+    f.size,
+    f.created,
+    f.modified,
+    f.thumbavif,
+    f.thumbx,
+    f.thumby,
+    f.meta
+FROM manifestattachment ma
+INNER JOIN [file] f ON f.id = ma.file_id
+INNER JOIN manifest m ON m.id = ma.manifest_id
+WHERE m.company_id = @companyId
+  AND m.[type] = @type
+ORDER BY ma.manifest_id, ma.sort ASC, ma.id ASC
+`;