S3 review: overview pool-cap check, membership re-read on pool_full (server-26#225)

- guild_overview offered an invite link to a full pool bot whenever it was
  in ANY server (`if member` tested the non-empty guild_ids list, not
  membership of this guild). Now: in this guild, or invitable.
- start_stream re-reads membership once on pool_full as it already does on
  no_bot_in_guild, so a full pool bot an admin just invited by hand is found.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/app/routers/discord.py

@@ -95,7 +95,7 @@ async def guild_overview(guild_id: str, _: dict = Depends(require_service_key_or
             "member": None if member is None else guild_id in member,
             # A pool bot at discord_pool_bot_guild_cap is never offered (#225, owner Q3).
             "invite_url": (leases.invite_url(t.get("application_id"), guild_id)
-                           if member or leases.invitable(t) else None),
+                           if guild_id in (member or []) or leases.invitable(t) else None),
         })
     return {
         "guild_id": guild_id,