refactor(community): give the provider port vendor-neutral types - #68

The port was generic in name only. Its methods returned DiscordGuildMembership,
DiscordMessage and DiscordReactionUser, and every identifier was validated
against the Discord snowflake format, so the identifier regex reached callers
that have no reason to know what a snowflake is. Replacing the provider would
have meant editing the interface and both api_v0 services rather than swapping
an adapter.

The port now speaks CommunityRole, CommunityMembership, CommunityMessage,
CommunityReactionUser and CommunityAccountProfile, treats identifiers as opaque
strings, and drops the Discord message length limit. Discord's snowflake
format, its 2000-character limit, and the global_name field it returns are
refinements applied in discord/schema.ts, which is the only layer that issues
those values. The provider-specific field name is mapped to displayName at the
adapter boundary, matching the provider_display_name column it is stored in.

Behaviour is unchanged: the adapter still rejects a malformed provider response
and a guild member response for another user, both of which depend on the
snowflake assertion that moved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>