ci: prune old releases, keeping the newest five

Releases were retained forever. Each build is ~121 MB of attachments, so the
Gitea server grew by that much on every push to main — five builds had already
reached 606 MB, and nothing would have stopped it.

The release job now deletes versioned releases beyond the newest five, and
their tags with them (a tag survives its release and would otherwise pile up).
Five keeps a previous build available for a friend or a bisect while staying
well under a gigabyte. The 'latest' pointer is explicitly excluded from
pruning: it is the launcher's feed, not a build.

.gitea/workflows/ci.yml

@@ -180,3 +180,31 @@ jobs:
             -Uri "$api/releases/$($pointer.id)/assets?name=manifest.json" `
             -Form @{ attachment = Get-Item bin/manifest.json } | Out-Null
           Write-Host "latest pointer now advertises $env:TAG"
+
+      - name: Prune old releases
+        shell: pwsh
+        env:
+          KEEP: '5'
+          TOKEN: ${{ secrets.GITEA_TOKEN }}
+        run: |
+          $ErrorActionPreference = 'Stop'
+          $api = "${{ github.server_url }}/api/v1/repos/${{ github.repository }}"
+          $headers = @{ Authorization = "token $env:TOKEN" }
+          $keep = [int]$env:KEEP
+
+          # Each build is ~121 MB of attachments, so without this the server
+          # grows by that much on EVERY push to main. Keep the newest $keep
+          # versioned releases: enough to grab a previous build or bisect a
+          # regression, bounded at well under a gigabyte.
+          $releases = Invoke-RestMethod -Method Get -Headers $headers -Uri "$api/releases?limit=100"
+          # Never touch the `latest` pointer — it is the launcher's feed, not a build.
+          $builds = @($releases | Where-Object { $_.tag_name -ne 'latest' } |
+            Sort-Object -Property created_at -Descending)
+
+          Write-Host "$($builds.Count) versioned release(s); keeping $keep"
+          foreach ($old in ($builds | Select-Object -Skip $keep)) {
+            Invoke-RestMethod -Method Delete -Headers $headers -Uri "$api/releases/$($old.id)" | Out-Null
+            # The tag survives its release and would otherwise accumulate.
+            Invoke-RestMethod -Method Delete -Headers $headers -Uri "$api/tags/$($old.tag_name)" -SkipHttpErrorCheck | Out-Null
+            Write-Host "  pruned $($old.tag_name)"
+          }