0268 — latest_key is one shared table across all topics, not per-topic

Context. Message logs are per-topic physical tables (message_log_<topic_id>) because they scale with message volume. latest_key holds exactly one row per (topic, key), so it scales with distinct-key count instead.

Decision. latest_key(topic_id, compaction_key, latest_id) is a single table shared across every topic, created by migration 006_latest_key — matching the shape of cursor/deliveries/binding, which are likewise shared and volume-independent, rather than message_log’s per-topic shape.

Consequences. No dynamic DDL on topic creation for this table; size stays bounded by live distinct keys regardless of republish count. Topic teardown must clean its latest_key rows like the other shared tables (the pre-existing DeleteTopic cleanup gap applies here identically — same gap, one more table).