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]

contracts/api.schemas.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://json-schema.org/draft/2020-12/schema",
-  "x-backend-contract-sha256": "340876061d7b324e4fd16972e276ffb0f238eeac66b214a043b3b4b98d560fcf",
+  "x-backend-contract-sha256": "e0ae14a47dfbb3c22017cdd7b5e12465b3e273842db2a35f36b7335c20b51cac",
   "$defs": {
     "AccountChannelBody": {
       "type": "object",
@@ -312,7 +312,7 @@
     "ChannelInfo": {
       "type": "object",
       "properties": {
-        "avatar": {
+        "avatar_version": {
           "anyOf": [
             {
               "type": "string"