feat(guild): join / cancel_join_request / join_request_list

Implements three join-path state machine endpoints:
- /guild/join: FREE (instant), APPROVAL (pending row + idempotent re-apply),
  ONLY_INVITE (pending invite required); CommitJoin consumes invites + cancels
  join requests; MaxMemberNum cap enforced.
- /guild/cancel_join_request: cancels all pending requests for caller (no
  request_id on wire per GuildJoinRequestCancelTask decompile, composite PK kept).
- /guild/join_request_list: leader/subleader sees pending applicants with
  viewer profile (name/emblem/degree/request_time unix secs).

IGuildService.CancelJoinRequestAsync signature changed from (viewerId, guildId)
to (viewerId) since the client sends no guildId. GuildJoinRequestEntry enriched
record added to GuildServiceTypes. Re-apply after cancel resets existing row
in-place to avoid composite-PK conflict. 11 integration tests, all green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Migrations/SVSimDbContextModelSnapshot.cs

@@ -198,7 +198,8 @@ namespace SVSim.Database.Migrations
                     b.HasKey("Id");
 
                     b.HasIndex("GuildId", "InviteeViewerId")
-                        .IsUnique();
+                        .IsUnique()
+                        .HasFilter("\"Status\" = 0");
 
                     b.HasIndex("InviteeViewerId", "Status");