Update ARCHITECTURE.md, bump version to 1.5.4, add CHANGELOG entry

CHANGELOG.md

@@ -6,6 +6,22 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
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+## [1.5.4] - 2026-05-19
+
+### Added
+
+- **Multi-select download client filter** — Active Downloads tab now includes a dropdown filter that allows users to select multiple download clients. Shows client name and type (e.g., "Main (qbitt)"). Includes Select All / Deselect All buttons. Selection persists across sessions via localStorage.
+- **Download client ordering** — Downloads are now sorted by client order (matching the configuration order). Downloads from the first configured client appear first.
+- **Client metadata in downloads** — Download objects now include `client`, `instanceId`, and `instanceName` fields for client identification and filtering.
+- **SSE payload extension** — SSE stream now includes `downloadClients` array with all configured clients for UI ordering/filtering.
+- **Automatic migration** — Existing single-select filter selection automatically migrates to new multi-select format on first load.
+
+### Fixed
+
+- **SABnzbd size and speed display** — Fixed SABnzbd downloads showing undefined size and speed values. Now correctly uses `slot.mb` for size calculation and `slot.kbpersec` for per-slot speed from cached data.
+
+---
+
 ## [1.5.3] - 2026-05-19
 
 ### Fixed