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/migrations/0040_compaction.up.sql

@@ -0,0 +1,28 @@
+-- 0040_compaction
+-- Add context compaction support: compaction logs table, compact_id on messages, bot settings columns.
+
+-- bot_history_message_compacts: stores compaction records and summaries.
+CREATE TABLE IF NOT EXISTS bot_history_message_compacts (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  bot_id UUID NOT NULL REFERENCES bots(id) ON DELETE CASCADE,
+  session_id UUID REFERENCES bot_sessions(id) ON DELETE SET NULL,
+  status TEXT NOT NULL DEFAULT 'pending' CHECK (status IN ('pending', 'ok', 'error')),
+  summary TEXT NOT NULL DEFAULT '',
+  message_count INTEGER NOT NULL DEFAULT 0,
+  error_message TEXT NOT NULL DEFAULT '',
+  usage JSONB,
+  model_id UUID REFERENCES models(id) ON DELETE SET NULL,
+  started_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  completed_at TIMESTAMPTZ
+);
+
+CREATE INDEX IF NOT EXISTS idx_compacts_bot_session ON bot_history_message_compacts(bot_id, session_id, started_at DESC);
+
+-- Add compact_id to messages to track which compaction a message belongs to.
+ALTER TABLE bot_history_messages ADD COLUMN IF NOT EXISTS compact_id UUID REFERENCES bot_history_message_compacts(id) ON DELETE SET NULL;
+CREATE INDEX IF NOT EXISTS idx_bot_history_messages_compact ON bot_history_messages(compact_id);
+
+-- Bot-level compaction settings.
+ALTER TABLE bots ADD COLUMN IF NOT EXISTS compaction_enabled BOOLEAN NOT NULL DEFAULT false;
+ALTER TABLE bots ADD COLUMN IF NOT EXISTS compaction_threshold INTEGER NOT NULL DEFAULT 100000;
+ALTER TABLE bots ADD COLUMN IF NOT EXISTS compaction_model_id UUID REFERENCES models(id) ON DELETE SET NULL;