feat: Add conversation merging functionality

[vats98754]

Implemented a comprehensive conversation merge feature that allows users to combine adjacent conversations with long-press selection and a merge toolbar.

Backend Changes:

• Added merge_conversations() function to database/conversations.py that:
  • Validates conversations are adjacent (max 1 hour gap)
  • Merges transcript segments, photos, action items, events, and audio files
  • Creates new merged conversation with combined data
  • Deletes source conversations after merge
• Added POST /v1/conversations/merge endpoint that:
  • Validates user permissions
  • Calls merge function
  • Reprocesses merged conversation to generate new title/overview/summary
  • Returns the processed merged conversation
• Added MergeConversationsRequest model for API request validation

Frontend Changes:

• Added selection mode state to ConversationProvider with:
  • isSelectionMode, selectedConversationIds, isMerging flags
  • Methods to enable/disable selection mode
  • toggleConversationSelection() to select/deselect conversations
  • canMergeSelectedConversations() to validate adjacency
  • mergeSelectedConversations() to call API and update UI
• Added mergeConversations() API method in conversations.dart
• Updated ConversationListItem widget to:
  • Handle long-press gesture to enable selection mode
  • Show checkboxes and blue border when in selection mode
  • Toggle selection on tap when in selection mode
• Added merge toolbar to ConversationsPage that:
  • Appears at bottom when selection mode is active
  • Shows selection count
  • Has Cancel button to exit selection mode
  • Has Merge button (disabled if not valid) to merge selected conversations
  • Shows loading indicator during merge operation
  • Displays success/error messages via SnackBar

Features:

• Long-press on any conversation to enter selection mode
• Select multiple conversations by tapping
• Only adjacent conversations (within 1 hour) can be merged
• Merge button disabled if selection is invalid
• Merged conversation gets auto-generated title and overview
• Original conversations are deleted after successful merge
• Visual feedback with checkboxes, borders, and loading states

Fixes #1505

[aaravgarg]

wow, can we see a demo?

really important for quick review @vats98754

[beastoin]

>> https://github.com/BasedHardware/omi/issues/1505#issuecomment-3578526884