Add Re-sync button for SimpleFIN connections

Resets LastSyncAt on all linked accounts so the next sync
re-fetches the full 90-day transaction window. Duplicate
detection prevents re-importing existing transactions.

- POST /api/bank-sync/connections/{id}/reset-sync endpoint
- Re-sync button shown only on SimpleFIN connection cards

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Api/Controllers/BankSyncController.cs

@@ -74,6 +74,13 @@ public class BankSyncController : ControllerBase
     public async Task<ActionResult<LinkedAccountResponse>> UpdateLinkedAccount(Guid id, UpdateLinkedAccountRequest request)
         => Ok(await _bankSyncService.UpdateLinkedAccountAsync(UserId, id, request));
 
+    [HttpPost("connections/{id}/reset-sync")]
+    public async Task<IActionResult> ResetSync(Guid id)
+    {
+        await _bankSyncService.ResetSyncDatesAsync(UserId, id);
+        return NoContent();
+    }
+
     [HttpPost("connections/{id}/sync-now")]
     public IActionResult SyncNow(Guid id)
     {