perf(host): store avatars as content-addressed files and serve them from disk

Space info now carries avatar_version instead of the inlined thumbnail; the worker fetches the thumbnail once per version. Legacy base64 rows are migrated to files on startup, and the daily backup uploads avatar files.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com> [CI]

web/src/api/generated.ts

@@ -1,5 +1,5 @@
 // Generated from backend DTOs; do not edit.
-// backend-contract-sha256: 340876061d7b324e4fd16972e276ffb0f238eeac66b214a043b3b4b98d560fcf
+// backend-contract-sha256: e0ae14a47dfbb3c22017cdd7b5e12465b3e273842db2a35f36b7335c20b51cac
 
 export interface AccountChannelBody {
   channel_id: string;
@@ -75,7 +75,7 @@ export interface ChannelHistoryRequest {
   filesHistory: boolean;
 }
 export interface ChannelInfo {
-  avatar?: string | null;
+  avatar_version?: string | null;
   can_create_channel: boolean;
   can_delete_any: boolean;
   can_delete_own: boolean;