feat: client-side ClamAV scanning for encrypted rooms (Issue #19)

Synapse's own check_media_file_for_spam module can never see E2EE
attachment content - only the client ever holds the decryption key.
Adds two hooks that call a self-hosted scan service
(https://axion1337.chat/_scan, deployed separately in the gitops repo):

- DecryptFile.ts: scans every decrypted attachment (image/audio/video/
  file all funnel through this one function via MediaEventHelper)
  before returning it as a Blob.
- ContentMessages.ts: scans plaintext before encryption/upload in
  uploadFile(), the shared function behind all attachment uploads
  (main file, thumbnails, voice messages), regardless of room
  encryption state.

New ContentScanRejectedError surfaces through the existing error-
rendering paths (MediaProcessingError, upload failure dialog) using
the same pattern as DecryptError/DownloadError/UploadFailedError.

Live-tested: EICAR blocked pre-upload in encrypted rooms and DMs;
receive-side hook also blocks EICAR sent by an unpatched client
(app.element.io), confirming it isn't just self-protection for our
own uploads. Fails open on scanner errors so an outage can't block
all uploads/downloads.

apps/web/src/viewmodels/message-body/FileBodyViewModel.ts

@@ -25,6 +25,7 @@ import { FileDownloader } from "../../utils/FileDownloader";
 import { type MediaEventHelper } from "../../utils/MediaEventHelper";
 import { TimelineRenderingType } from "../../contexts/RoomContext";
 import ErrorDialog from "../../components/views/dialogs/ErrorDialog";
+import { ContentScanRejectedError } from "../../utils/ContentScanner";
 
 export interface FileBodyViewModelProps {
     mxEvent: MatrixEvent;
@@ -249,7 +250,10 @@ export class FileBodyViewModel
             logger.warn("Unable to decrypt attachment: ", err);
             Modal.createDialog(ErrorDialog, {
                 title: _t("common|error"),
-                description: _t("timeline|m.file|error_decrypting"),
+                description:
+                    err instanceof ContentScanRejectedError
+                        ? _t("timeline|m.file|error_scan_rejected")
+                        : _t("timeline|m.file|error_decrypting"),
             });
         }
     };

apps/web/src/viewmodels/message-body/VideoBodyViewModel.ts

@@ -23,6 +23,7 @@ import { mediaFromContent } from "../../customisations/Media";
 import { BLURHASH_FIELD } from "../../utils/image-media";
 import { type ImageSize, suggestedSize as suggestedVideoSize } from "../../settings/enums/ImageSize";
 import { type MediaEventHelper } from "../../utils/MediaEventHelper";
+import { ContentScanRejectedError } from "../../utils/ContentScanner";
 
 export interface VideoBodyViewModelProps {
     /**
@@ -203,7 +204,10 @@ export class VideoBodyViewModel
         if (state.error !== null) {
             return {
                 state: VideoBodyViewState.ERROR,
-                errorLabel: _t("timeline|m.video|error_decrypting"),
+                errorLabel:
+                    state.error instanceof ContentScanRejectedError
+                        ? _t("timeline|m.video|error_scan_rejected")
+                        : _t("timeline|m.video|error_decrypting"),
                 maxWidth,
                 maxHeight,
                 aspectRatio,