docs: add ETL pipeline overview documentation

DOCUMENTATION/DataSync/Overview.md

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+# ETL Pipeline
+
+The ETL pipeline streams data from enterprise sources (JDE, CMS) through transformations into SQL Server cache tables. It supports batched processing, pre/post scripts for index management, and detailed execution tracking.
+
+## Architecture
+
+```
+┌─────────────┐    ┌──────────────────┐    ┌─────────────────┐
+│ IImportSource│───▶│ IDataTransformer │───▶│IImportDestination│
+└─────────────┘    │   (chain of N)   │    └─────────────────┘
+                   └──────────────────┘
+       ▲                                           │
+       │         ┌──────────────┐                  │
+       └─────────│ Pre-Scripts  │                  ▼
+                 └──────────────┘           ┌──────────────┐
+                                            │ Post-Scripts │
+                                            └──────────────┘
+```
+
+**Execution flow:**
+1. Run pre-scripts (e.g., disable indexes)
+2. Open source and get `IDataReader`
+3. Apply transformer chain (each wraps the previous reader)
+4. Write to destination (bulk copy or merge)
+5. Run post-scripts (e.g., rebuild indexes)
+
+## Core Contracts
+
+### IImportSource
+
+Provides data to the pipeline. Returns an `IDataReader` that streams rows.
+
+```csharp
+public interface IImportSource : IAsyncDisposable
+{
+    Task<IDataReader> ReadDataAsync(CancellationToken cancellationToken = default);
+    string SourceName { get; }
+}
+```
+
+### IDataTransformer
+
+Modifies data during transfer. Wraps the source reader in a decorator.
+
+```csharp
+public interface IDataTransformer
+{
+    IDataReader Transform(IDataReader source);
+    string TransformerName { get; }
+    int MapOrdinal(int transformedOrdinal, IDataReader source);
+}
+```
+
+### IImportDestination
+
+Consumes data and writes to storage. Returns statistics about the operation.
+
+```csharp
+public interface IImportDestination
+{
+    Task<DestinationResult> WriteAsync(IDataReader source, CancellationToken cancellationToken = default);
+    string DestinationName { get; }
+}
+```
+
+### IScriptRunner
+
+Executes SQL scripts before or after data transfer.
+
+```csharp
+public interface IScriptRunner
+{
+    Task ExecuteAsync(CancellationToken cancellationToken = default);
+    string ScriptName { get; }
+}
+```
+
+## Pipeline Execution
+
+The `EtlPipeline` class orchestrates execution and tracks timing for each step:
+
+```csharp
+public async Task<PipelineResult> ExecuteAsync(CancellationToken cancellationToken = default)
+{
+    // 1. Run pre-scripts
+    foreach (var script in _preScripts)
+    {
+        var stepResult = await RunScriptAsync(script, cancellationToken);
+        steps.Add(stepResult);
+    }
+
+    // 2. Open source
+    await using (_source)
+    {
+        var reader = await _source.ReadDataAsync(cancellationToken);
+
+        // 3. Apply transformers
+        foreach (var transformer in _transformers)
+        {
+            reader = transformer.Transform(reader);
+        }
+
+        // 4. Write to destination
+        var destResult = await _destination.WriteAsync(reader, cancellationToken);
+    }
+
+    // 5. Run post-scripts
+    foreach (var script in _postScripts)
+    {
+        var stepResult = await RunScriptAsync(script, cancellationToken);
+    }
+
+    return PipelineResult.Succeeded(totalRows, totalStopwatch.Elapsed, steps);
+}
+```
+
+## Result Model
+
+### PipelineResult
+
+```csharp
+public record PipelineResult(
+    bool Success,
+    long TotalRows,
+    TimeSpan Elapsed,
+    IReadOnlyList<StepResult> Steps,
+    Exception? Error = null);
+```
+
+### StepResult
+
+```csharp
+public record StepResult(
+    string StepName,
+    string StepType,
+    long RowsAffected,
+    TimeSpan Elapsed);
+```
+
+### DestinationResult
+
+```csharp
+public record DestinationResult(
+    long RowsProcessed,
+    int BatchCount,
+    TimeSpan Elapsed);
+```
+
+## Related Documentation
+
+- [Sources](./Sources.md) - Writing custom data sources
+- [Transformers](./Transformers.md) - Writing custom transformers
+- [Destinations](./Destinations.md) - Writing destinations and scripts
+- [Configuration](./Configuration.md) - Pipeline builder and DI setup
+- [Troubleshooting](./Troubleshooting.md) - Debugging and performance