Port the forum domain contexts #18

Outcome

The Effect forum domain (workers/api/src/forum/ in the openagents monorepo) is ported to Elixir contexts: OpenAgents.Forum for topics and posts.

Work

• Topic and post creation, listing, pagination, editing, soft deletion.
• Port the rate-limit policy where it still applies.
• Activity records for every committed write so other surfaces can subscribe.
• Do not port tip/settlement code: payments were retired before the forum's last change. Record that decision here.

Acceptance criteria

• Context functions cover every operation the TanStack routes exercised.
• Domain unit tests ported or rewritten.

Verification

Context test suite; mix precommit.