feat: migrate message insert paths to ciphertext + envelopes

[aizyohiy-hash]

Summary

This PR updates the messaging pipeline to support encrypted message storage and removes the deprecated server-side full-text search endpoint as part of the migration.

Changes Made

• Migrated all message insert paths to use ciphertext and encrypted message envelopes.
• Updated the message persistence flow to store encrypted payloads instead of plaintext.
• Removed server-side full-text search functionality.
• Updated the deprecated search endpoint to return HTTP 410 Gone with a migration note indicating that the feature is no longer supported.
• Aligned the messaging API with the new encrypted data model and migration strategy.

Benefits

• Strengthens end-to-end security by ensuring message content is stored as encrypted ciphertext.
• Simplifies the backend by removing deprecated plaintext search functionality.
• Provides a clear migration path for clients through a standardized 410 Gone response.
• Supports the application's transition to a more secure, privacy-focused messaging architecture.

Closed #174
Closed #180

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