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/compaction/types.go

@@ -0,0 +1,33 @@
+package compaction
+
+import "time"
+
+// Log represents a compaction log entry.
+type Log struct {
+	ID           string     `json:"id"`
+	BotID        string     `json:"bot_id"`
+	SessionID    string     `json:"session_id,omitempty"`
+	Status       string     `json:"status"`
+	Summary      string     `json:"summary"`
+	MessageCount int        `json:"message_count"`
+	ErrorMessage string     `json:"error_message"`
+	Usage        any        `json:"usage,omitempty"`
+	ModelID      string     `json:"model_id,omitempty"`
+	StartedAt    time.Time  `json:"started_at"`
+	CompletedAt  *time.Time `json:"completed_at,omitempty"`
+}
+
+// ListLogsResponse is the API response for listing compaction logs.
+type ListLogsResponse struct {
+	Items []Log `json:"items"`
+}
+
+// TriggerConfig holds the parameters needed to trigger a compaction.
+type TriggerConfig struct {
+	BotID      string
+	SessionID  string
+	ModelID    string
+	ClientType string
+	APIKey     string //nolint:gosec // runtime credential, not a hardcoded secret
+	BaseURL    string
+}