feat: migrate messages content to ciphertext (#173)

[Gabugo-tech]

Description

This PR migrates the messages table and its dependent codebase to use E2E encrypted ciphertext payloads instead of plain text content. This prepares the backend for group/MLS single-ciphertext messages where the server can no longer read the contents of messages directly.

Changes Made:

• Database Schema (apps/backend/src/db/schema.ts):
  • Dropped the content column in the messages table.
  • Added a nullable ciphertext column.
  • Removed the messages_content_search_idx Gin index that was built for full-text search on plain text content.
• Message Serialization (apps/backend/src/lib/messages.ts):
  • Updated serializeMessage to accept and correctly handle ciphertext.
• API & Routes (apps/backend/src/routes/conversations.ts):
  • Removed the GET /:id/search full-text search endpoint since the backend can no longer query plain text message content.
• Websockets Integration (apps/backend/src/socket/messaging.ts):
  • Replaced content payloads with ciphertext in send_message and ask_assistant event handlers.
• Testing:
  • Migrated all mock values and testing payloads to use ciphertext instead of content.
  • Deleted full-text search endpoint unit tests.

Related Issues

• Resolves Add messages.ciphertext for single-blob messages + remove plaintext content #173

Acceptance Criteria

• content column dropped; ciphertext added (nullable)
• serializeMessage no longer references plaintext
• Message insert paths and sockets updated
• Search removal coordinated
• Application successfully compiles and all tests pass

closes #173

[drips-wave]

@Gabugo-tech Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[codebestia]

@Gabugo-tech
Please fix the CI