Forum schema and migrations #17

Outcome

Postgres migrations in the Phoenix app define forum_topics, forum_posts, and identity-link tables, matching the semantics of the live Effect forum schema in khala_sync_prod.

Work

• Ecto schemas for topics and posts: ids, refs, author identity, markdown body, timestamps, soft-delete state.
• Preserve legacy ids (topicId, post refs) as columns so migrated content keeps its URL structure.
• Identity-link table mapping a new OpenAgents account to a legacy forum actor id plus proof-of-ownership state.
• Indexes for topic listing, per-topic post paging, and author lookups.

Acceptance criteria

• Migrations run green on a fresh database.
• Schema round-trips a sample row from khala_sync_prod without loss.

Verification

Migration tests and a round-trip fixture test; mix precommit.