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/compaction_logs.sql

@@ -0,0 +1,38 @@
+-- name: CreateCompactionLog :one
+INSERT INTO bot_history_message_compacts (bot_id, session_id)
+VALUES ($1, $2)
+RETURNING id, bot_id, session_id, status, summary, message_count, error_message, usage, model_id, started_at, completed_at;
+
+-- name: CompleteCompactionLog :one
+UPDATE bot_history_message_compacts
+SET status = $2,
+    summary = $3,
+    message_count = $4,
+    error_message = $5,
+    usage = $6,
+    model_id = $7,
+    completed_at = now()
+WHERE id = $1
+RETURNING id, bot_id, session_id, status, summary, message_count, error_message, usage, model_id, started_at, completed_at;
+
+-- name: GetCompactionLogByID :one
+SELECT id, bot_id, session_id, status, summary, message_count, error_message, usage, model_id, started_at, completed_at
+FROM bot_history_message_compacts
+WHERE id = $1;
+
+-- name: ListCompactionLogsByBot :many
+SELECT id, bot_id, session_id, status, summary, message_count, error_message, usage, model_id, started_at, completed_at
+FROM bot_history_message_compacts
+WHERE bot_id = $1
+  AND ($2::timestamptz IS NULL OR started_at < $2)
+ORDER BY started_at DESC
+LIMIT $3;
+
+-- name: ListCompactionLogsBySession :many
+SELECT id, bot_id, session_id, status, summary, message_count, error_message, usage, model_id, started_at, completed_at
+FROM bot_history_message_compacts
+WHERE session_id = $1
+ORDER BY started_at ASC;
+
+-- name: DeleteCompactionLogsByBot :exec
+DELETE FROM bot_history_message_compacts WHERE bot_id = $1;