feat: SyncQueue table and set-based apply stored procedure

- sql/01_sync_queue.sql: idempotent DDL for dbo.SyncQueue (PK + unique
  dedup index + pending lookup index), safe to re-run.
- sql/02_sync_apply.sql: dbo.usp_SyncApply (@MaxRetries INT=5). Per
  distinct (TargetSchema,TargetTable) it builds column projections from
  sys.columns (excludes ID key/computed/identity/rowversion) and runs a
  dynamic-SQL MERGE (last-write-wins via ROW_NUMBER over SourceLogID DESC)
  for Insert/Update plus a DELETE for the last op = Delete.
  SET IDENTITY_INSERT ON preserves Access PKs.
- tests/conftest.py: sql_conn fixture reads conn_str from gitignored
  config.yaml via load_config; skipped without RUN_INTEGRATION=1.
- tests/test_apply_proc.py: integration test covering IDENTITY-preserving
  INSERT, last-write-wins UPDATE, BIT conversion, and DELETE; cleans up.

Deviation from the brief's procedure (root-cause fix, design preserved):
every JSON path key is quoted ('$."col"') so non-ASCII column names
(e.g. Chinese 名字/数量) parse correctly. Without quoting, JSON_VALUE
raises "JSON path format is not correct" on Chinese columns, which is the
real target schema for this Access->SQL Server sync.

Co-Authored-By: Claude <noreply@anthropic.com>

sql/02_sync_apply.sql

@@ -0,0 +1,115 @@
+-- usp_SyncApply: set-based apply of pending SyncQueue rows.
+-- Per distinct (TargetSchema, TargetTable) it builds column lists from sys.columns
+-- (excluding the ID key, computed, identity and rowversion columns) and runs a
+-- dynamic-SQL MERGE (last-write-wins by SourceLogID DESC) for Insert/Update and a
+-- DELETE for the last op = Delete. SET IDENTITY_INSERT ON preserves Access PKs.
+
+CREATE OR ALTER PROCEDURE dbo.usp_SyncApply
+    @MaxRetries INT = 5
+AS
+BEGIN
+    SET NOCOUNT ON;
+
+    DECLARE @sch NVARCHAR(128), @tbl NVARCHAR(255), @FullName NVARCHAR(514);
+    DECLARE @sql NVARCHAR(MAX), @cols NVARCHAR(MAX), @upd NVARCHAR(MAX), @ins NVARCHAR(MAX);
+
+    -- Re-queue rows still under the retry budget.
+    UPDATE dbo.SyncQueue
+       SET Status = 'pending'
+     WHERE Status = 'error' AND RetryCount < @MaxRetries;
+
+    DECLARE cur CURSOR LOCAL FAST_FORWARD FOR
+        SELECT DISTINCT TargetSchema, TargetTable
+          FROM dbo.SyncQueue
+         WHERE Status = 'pending';
+
+    OPEN cur;
+    FETCH NEXT FROM cur INTO @sch, @tbl;
+
+    WHILE @@FETCH_STATUS = 0
+    BEGIN
+        SET @FullName = QUOTENAME(@sch) + N'.' + QUOTENAME(@tbl);
+
+        BEGIN TRY
+            BEGIN TRAN;
+            -- @cols: comma-quoted column names (for INSERT target list)
+            -- @upd:  tgt.col = JSON_VALUE(src.RowData,'$.col')
+            -- @ins:  JSON_VALUE(src.RowData,'$.col')  (for INSERT VALUES)
+            -- NOTE: every JSON path key is wrapped in double quotes ('$."col"').
+            -- This is mandatory for non-ASCII column names (e.g. Chinese names
+            -- like 名字) and harmless for ASCII names, so we quote unconditionally.
+            SELECT
+                @cols = STRING_AGG(QUOTENAME(c.name), N',')
+                            WITHIN GROUP (ORDER BY c.column_id),
+                @upd  = STRING_AGG(QUOTENAME(c.name) + N'=JSON_VALUE(src.RowData,''$."' + c.name + N'"'')', N',')
+                            WITHIN GROUP (ORDER BY c.column_id),
+                @ins  = STRING_AGG(N'JSON_VALUE(src.RowData,''$."' + c.name + N'"'')', N',')
+                            WITHIN GROUP (ORDER BY c.column_id)
+              FROM sys.columns c
+             WHERE c.object_id = OBJECT_ID(@FullName)
+               AND c.is_computed = 0
+               AND c.is_identity = 0
+               AND TYPE_NAME(c.system_type_id) <> 'timestamp'
+               AND c.name <> 'ID';
+
+            IF @cols IS NOT NULL
+            BEGIN
+                -- Upsert: last write wins (ROW_NUMBER over SourceLogID DESC).
+                SET @sql = N'SET IDENTITY_INSERT ' + @FullName + N' ON;'
+                         + N'MERGE ' + @FullName + N' WITH (HOLDLOCK) AS tgt'
+                         + N' USING ('
+                         + N'  SELECT RecordID, RowData FROM ('
+                         + N'    SELECT *, ROW_NUMBER() OVER (PARTITION BY RecordID ORDER BY SourceLogID DESC) rn'
+                         + N'      FROM dbo.SyncQueue'
+                         + N'     WHERE TargetSchema=@sch AND TargetTable=@tbl AND Status=''pending'''
+                         + N'       AND OperateType IN (''Insert'',''Update'') AND RowData IS NOT NULL'
+                         + N'  ) x WHERE rn=1'
+                         + N') AS src ON tgt.ID = TRY_CAST(src.RecordID AS int)'
+                         + N' WHEN MATCHED THEN UPDATE SET ' + @upd
+                         + N' WHEN NOT MATCHED THEN INSERT (ID,' + @cols + N')'
+                         + N'  VALUES (TRY_CAST(src.RecordID AS int),' + @ins + N');'
+                         + N'SET IDENTITY_INSERT ' + @FullName + N' OFF;';
+                EXEC sp_executesql @sql,
+                     N'@sch NVARCHAR(128),@tbl NVARCHAR(255)', @sch, @tbl;
+
+                -- Delete: last op = Delete wins.
+                SET @sql = N'DELETE t FROM ' + @FullName + N' t'
+                         + N' JOIN ('
+                         + N'  SELECT RecordID FROM ('
+                         + N'    SELECT *, ROW_NUMBER() OVER (PARTITION BY RecordID ORDER BY SourceLogID DESC) rn'
+                         + N'      FROM dbo.SyncQueue'
+                         + N'     WHERE TargetSchema=@sch AND TargetTable=@tbl AND Status=''pending'''
+                         + N'       AND OperateType=''Delete'''
+                         + N'  ) x WHERE rn=1'
+                         + N') d ON t.ID = TRY_CAST(d.RecordID AS int);';
+                EXEC sp_executesql @sql,
+                     N'@sch NVARCHAR(128),@tbl NVARCHAR(255)', @sch, @tbl;
+            END
+
+            UPDATE dbo.SyncQueue
+               SET Status = 'applied', AppliedAt = SYSDATETIME()
+             WHERE TargetSchema = @sch AND TargetTable = @tbl AND Status = 'pending';
+
+            COMMIT;
+        END TRY
+        BEGIN CATCH
+            -- ROLLBACK unwinds the per-table transaction (and, if the caller
+            -- started one, the caller's too). A proc that errors under an outer
+            -- transaction is expected to leave the txn uncommittable; the caller
+            -- issues its own rollback. We roll back here so the partial per-table
+            -- work is discarded before marking rows as 'error'/'dead'.
+            IF @@TRANCOUNT > 0 ROLLBACK;
+            UPDATE dbo.SyncQueue
+               SET Status      = CASE WHEN RetryCount + 1 >= @MaxRetries THEN 'dead' ELSE 'error' END,
+                   RetryCount   = RetryCount + 1,
+                   ErrorMsg     = ERROR_MESSAGE()
+             WHERE TargetSchema = @sch AND TargetTable = @tbl AND Status = 'pending';
+        END CATCH
+
+        FETCH NEXT FROM cur INTO @sch, @tbl;
+    END
+
+    CLOSE cur;
+    DEALLOCATE cur;
+END
+GO