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

internal/db/sqlc/conversations.sql.go

@@ -511,7 +511,7 @@ WITH updated AS (
   SET display_name = $1,
       updated_at = now()
   WHERE bots.id = $2
-  RETURNING id, owner_user_id, display_name, avatar_url, is_active, status, max_context_load_time, max_context_tokens, language, reasoning_enabled, reasoning_effort, chat_model_id, search_provider_id, memory_provider_id, heartbeat_enabled, heartbeat_interval, heartbeat_prompt, heartbeat_model_id, title_model_id, tts_model_id, browser_context_id, metadata, created_at, updated_at
+  RETURNING id, owner_user_id, display_name, avatar_url, is_active, status, max_context_load_time, max_context_tokens, language, reasoning_enabled, reasoning_effort, chat_model_id, search_provider_id, memory_provider_id, heartbeat_enabled, heartbeat_interval, heartbeat_prompt, heartbeat_model_id, title_model_id, tts_model_id, browser_context_id, compaction_enabled, compaction_threshold, compaction_model_id, metadata, created_at, updated_at
 )
 SELECT
   updated.id AS id,