feat: add conversation storage types

Add type definitions for the new conversation storage system:
- ConversationMessage: Individual message structure with tool calls/results
- ConversationTableMeta: Metadata for per-user conversation tables
- ConversationStats: Aggregated statistics per user
- ArchiveResult: Results from archiving operations

These types will be used by the upcoming conversation storage implementation
that replaces the message_buffer + summaries system with dynamic per-user
tables and automatic archiving.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types.ts

@@ -787,3 +787,39 @@ export interface Message<T> {
   ack(): void;
   retry(): void;
 }
+
+// ============================================
+// Conversation Storage Types
+// ============================================
+
+export interface ConversationMessage {
+  id?: number;
+  role: 'user' | 'assistant';
+  content: string;
+  tool_calls?: string;    // JSON: [{name, arguments, id}]
+  tool_results?: string;  // JSON: [{tool_call_id, result}]
+  created_at?: string;
+}
+
+export interface ConversationTableMeta {
+  telegram_id: string;
+  table_name: string;
+  message_count: number;
+  last_message_at: string | null;
+  created_at: string;
+}
+
+export interface ConversationStats {
+  telegram_id: string;
+  message_count: number;
+  first_message_at: string | null;
+  last_message_at: string | null;
+  archived_summaries: number;
+}
+
+export interface ArchiveResult {
+  processed_users: number;
+  archived_messages: number;
+  created_summaries: number;
+  errors: string[];
+}