feat(lexicon): apply atProto best practices to community lexicons

Applied atProto best practices from https://github.com/bluesky-social/atproto/discussions/4245

Changes:
- getMembers.json: Changed 'enum' to 'knownValues' for sort parameter to allow schema evolution
- profile.json: Removed 'visibility' and 'moderationType' from required fields (both have defaults)

These changes improve forward compatibility and follow atProto lexicon design guidelines
while maintaining full backward compatibility with existing code.

Impact: Zero breaking changes - all tests pass, service layer already handles defaults.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

internal/atproto/lexicon/social/coves/community/getMembers.json

···
           },
           "sort": {
             "type": "string",
-            "enum": ["reputation", "recent", "alphabetical"],
-            "default": "reputation"
+            "knownValues": ["reputation", "recent", "alphabetical"],
+            "default": "reputation",
+            "maxLength": 64
           }
         }
       },

internal/atproto/lexicon/social/coves/community/profile.json

···
       "key": "literal:self",
       "record": {
         "type": "object",
-        "required": ["handle", "name", "createdAt", "createdBy", "hostedBy", "visibility", "moderationType"],
+        "required": ["handle", "name", "createdAt", "createdBy", "hostedBy"],
         "properties": {
           "handle": {
             "type": "string",