feat: add context compaction to automatically summarize old messages (#compaction) (#276)

When input tokens exceed a configurable threshold after a conversation round,
the system asynchronously compacts older messages into a summary. Cascading
compactions reference prior summaries via <prior_context> tags to maintain
conversational continuity without duplicating content.

- Add bot_history_message_compacts table and compact_id on messages
- Add compaction_enabled, compaction_threshold, compaction_model_id to bots
- Implement compaction service (internal/compaction) with LLM summarization
- Integrate into conversation flow: replace compacted messages with summaries
  wrapped in <summary> tags during context loading
- Add REST API endpoints (GET/DELETE /bots/:bot_id/compaction/logs)
- Add frontend Compaction tab with settings and log viewer
- Wire compaction service into both dev (cmd/agent) and prod (cmd/memoh) entry points
- Update test mocks to include new GetBotByID columns

db/queries/messages.sql

@@ -148,6 +148,7 @@ SELECT
   m.content,
   m.metadata,
   m.usage,
+  m.compact_id,
   m.created_at,
   ci.display_name AS sender_display_name,
   ci.avatar_url AS sender_avatar_url,
@@ -173,6 +174,7 @@ SELECT
   m.content,
   m.metadata,
   m.usage,
+  m.compact_id,
   m.created_at,
   ci.display_name AS sender_display_name,
   ci.avatar_url AS sender_avatar_url,
@@ -360,3 +362,16 @@ WHERE m.bot_id = sqlc.arg(bot_id)
   ) ILIKE '%' || sqlc.narg(keyword)::text || '%')
 ORDER BY m.created_at DESC
 LIMIT sqlc.arg(max_count);
+
+-- name: MarkMessagesCompacted :exec
+UPDATE bot_history_messages
+SET compact_id = $1
+WHERE id = ANY($2::uuid[]);
+
+-- name: ListUncompactedMessagesBySession :many
+SELECT id, chat_id, session_id, role, content, usage, platform, external_message_id, sender_channel_identity_id, compact_id, created_at
+FROM bot_history_messages
+WHERE session_id = $1
+  AND compact_id IS NULL
+  AND is_active = true
+ORDER BY created_at ASC;