add a button to clear out finished share links

The dashboard listed every share ever made, including ones revoked or expired
months ago, and nothing ever pruned them. There is now a "Clean up finished links"
button next to Refresh that runs a normal cleanup pass and then drops the records
that are done with, reporting how many went.

Only revoked, expired and failed records are removed. A link that can still be
used is left alone, including a spent one-use link whose guest is still watching
until it expires, since that record is Redeemed rather than finished.

Folded into 1.0.3.0 rather than shipped as 1.0.3.1: the ceiling fix and this are
going out as one replacement of that version.

Jellyfin.Plugin.ShareLinks/Api/ShareLinksController.cs

@@ -269,6 +269,21 @@ public sealed class ShareLinksController : ControllerBase
         return Ok(ToDto(record));
     }
 
+    /// <summary>Removes revoked, expired and failed share links from the store.</summary>
+    [HttpPost("Admin/Cleanup")]
+    [Authorize(AuthenticationSchemes = "CustomAuthentication")]
+    public async Task<ActionResult> Cleanup(CancellationToken cancellationToken)
+    {
+        SetNoStoreHeaders();
+        if (!User.IsInRole("Administrator"))
+        {
+            return Forbid();
+        }
+
+        var removed = await _cleanupService.PurgeFinishedAsync(cancellationToken).ConfigureAwait(false);
+        return Ok(new { removed });
+    }
+
     /// <summary>Returns the guest session state for the current authenticated user.</summary>
     [HttpGet("GuestState")]
     [Authorize(AuthenticationSchemes = "CustomAuthentication")]