Implement ConversationStorageService with Local Database

[FJiangArthur]

Priority: Medium
Phase: 4
Estimate: 4 story points

Description:
Implement local database storage for conversations, transcriptions, and analysis results using SQLite.

Current Issues:

• No persistent storage for conversations
• Conversations are lost when app restarts
• No conversation history or search capability

Acceptance Criteria:

• SQLite database is properly configured
• Conversations are stored with complete metadata
• Transcription segments are persisted with timestamps
• Analysis results are linked to conversations
• Database migrations are handled properly

Implementation Location:
lib/services/implementations/conversation_storage_service_impl.dart