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

ARCHITECTURE.md

@@ -460,6 +460,21 @@ When a browser opens `GET /api/dashboard/stream`:
 
 The browser's native `EventSource` API handles reconnection automatically on network interruption.
 
+**SSE Payload Structure**
+
+```javascript
+{
+  user: string,              // Username
+  isAdmin: boolean,          // Admin flag
+  downloads: DownloadObject[],  // Matched download objects (see Section 5.4)
+  downloadClients: {         // Configured download clients for ordering/filtering
+    id: string,              // Instance identifier
+    name: string,            // Instance display name
+    type: string             // Client type ('sabnzbd', 'qbittorrent', 'transmission', 'rtorrent')
+  }[]
+}
+```
+
 ### 5.4 Download Matching Pipeline
 
 For each connected user the server:
@@ -495,6 +510,14 @@ Users are matched to downloads via Sonarr/Radarr tags:
 1. **Exact match** — tag label (lowercased) === username (lowercased).
 2. **Sanitised match** — handles Ombi tag mangling: `sanitizeTagLabel()` converts to lowercase, replaces non-alphanumeric chars with hyphens, collapses runs, trims.
 
+#### Client ordering and filtering
+
+Matched download objects include `client`, `instanceId`, and `instanceName` fields. The frontend:
+1. Receives a `downloadClients` array from the SSE payload with all configured clients in configuration order
+2. Displays a multi-select filter allowing users to choose which clients to view
+3. Sorts downloads by client order (downloads from the first configured client appear first)
+4. Filters downloads to show only those from selected client instances
+
 #### Matched download object fields
 
 | Field | Type | Description |
@@ -523,6 +546,9 @@ Users are matched to downloads via Sonarr/Radarr tags:
 | `arrInstanceKey` | string/null | (Admin) API key for the *arr instance |
 | `arrContentId` | number/null | (Admin) `episodeId` or `movieId` for triggering a new search |
 | `arrContentType` | `'episode'`/`'movie'`/null | (Admin) Content type for search command |
+| `client` | string | Download client type ('sabnzbd', 'qbittorrent', 'transmission', 'rtorrent') |
+| `instanceId` | string | Instance identifier matching the configured client ID |
+| `instanceName` | string | Instance display name from configuration |
 | `addedOn` | number/null | (qBittorrent) Unix timestamp when torrent was added |
 | `availableForUpgrade` | boolean/undefined | (History) `true` when outcome is `failed` but content is on disk |
 
@@ -683,7 +709,7 @@ stateDiagram-v2
 | `handleLogin()` | Authenticate, fade login → splash → dashboard |
 | `startSSE()` | Open `EventSource` to `/stream`; handle incoming data |
 | `stopSSE()` | Close `EventSource` and cancel reconnect timer |
-| `renderDownloads()` | Diff-based card rendering (create/update/remove) |
+| `renderDownloads()` | Diff-based card rendering (create/update/remove); filters by selected download clients; sorts by client order |
 | `createDownloadCard()` | Build DOM for a single card; renders tag badges, import-issue badge, blocklist button |
 | `updateDownloadCard()` | Update existing card in-place (progress, speed, etc.) |
 | `handleBlocklistSearch()` | Confirm dialog → POST `/blocklist-search` → update button state |
@@ -698,6 +724,22 @@ stateDiagram-v2
 - **Regular user view** — a single accent-coloured badge showing the tag label that matched the current user's username (via `matchedUserTag`).
 - **Admin `showAll` view** — all tags on the download are rendered using `tagBadges[]`: tags with no matching Emby user → amber badge (leftmost); tags matched to a known Emby user → accent badge showing the Emby display name (rightmost).
 
+#### Download Client Filter
+
+The Active Downloads tab includes a multi-select dropdown filter that allows users to:
+- View all download clients with their type displayed as "Client Name (type)"
+- Select multiple clients to filter the downloads list
+- Use "Select All" / "Deselect All" buttons for bulk operations
+- Persist selection across sessions via localStorage
+
+Downloads are sorted by client order (matching the configuration order) and filtered by the selected client IDs.
+
+Related functions:
+- `initDownloadClientFilter()` — Sets up dropdown toggle, click-outside handler, Select/Deselect All buttons
+- `updateDownloadClientFilter()` — Populates checkbox list with client name + type badges
+- `toggleClientSelection()` — Updates selection array and localStorage
+- `updateSelectedCountDisplay()` — Updates button text to show "All clients" / "1 selected" / "N selected"
+
 ---
 
 ## 8. Directory Structure